6 Principles to Classical Theism

Below is an AI Generated Report on 6 crucial principles the classical theistic tradition is built on. I struggled to find a place where all these principles, and their principled arguments, are all in one place. Thankfully AI was able to do a good job and provide some quality sources to follow up on. Enjoy:

Introduction and Roadmap

In classical theism, the ultimate transmundane reality (God) is understood as absolutely simple, purely actual, uniquely existent, and the one cause of all (without composition, change, or participation in any higher category). Going forward this piece will treat each of the six principles below as both independent arguments and interlocking parts of a network. The six are: (1) the first principle is pure actuality (no unrealized potency); (2) any many-part thing has a cause of its unity; (3) every composite is posterior to and caused by its parts, so the first must be simple; (4) the first principle is not contained in any genus; (5) the Necessary Existent has no quiddity other than existence (so is not composite or in a genus); and (6) asserting multiple essential attributes of God destroys divine unity, so God is one simple substance. We will show each by close reading of primary texts (Aristotle, Aquinas, Avicenna, Maimonides), reconstructing their arguments, and then explain how they mutually support one another.

To clarify terminology: Classical theism here denotes a non-confessional approach tracing from Aristotle through medieval thinkers the idea of God as the absolutely first cause – an unmoved mover or Necessary Being. (Key terms: Aristotle’s energeia (actuality) vs. dunamis (potentiality); Aquinas’s actus vs. potentia; Avicenna’s wājib al-wujūd (necessary being), māhiyya (quiddity), inniyya/anniyya (individual essence).) We adopt a neutral voice, focusing on argument rather than creed. The essay below will argue that each principle is demanded by core texts and concepts, and together they entail the classical God’s simplicity, necessary existence, and unity. In the conclusion, this piece sketches how this framework yields familiar divine attributes (immutability, eternity, etc.) without appealing to revelation. This is the rational core to classical theism, if you will, at least for a core 6 principles. Obviously, the classical tradition is much deeper than this, this is a helpful start, however. Enjoy!

This roadmap sets out each section. We begin with Pure Actuality (I.1), explaining why an ultimate cause must have no potential. Then Cause of Unity (I.2) analyzes Aristotle’s view that any composite whole requires a cause beyond its parts to make it one. Next Simplicity from Non-Posteriority (I.3) shows Aquinas’s famous argument that since composites are posterior and caused but the first is uncaused, it cannot be composite. Not in a Genus (I.4) examines Aristotle’s and Aquinas’s point that being itself cannot be a genus, so the first principle has no genus. Principle 5 treats Avicenna’s Necessary Existent, who argues that God’s essence is pure existence, entailing no quiddity or genus. Principle 6 uses Maimonides on Attributes, concluding that God’s unity forces Him to be one simple substance (any plurality of attributes would yield multiple beings). Then a Synthesis shows how these points interlock into a single picture: the first principle emerges as pure act, simple, non-genus – meeting the adequacy criterion. Finally we briefly address Objections (modal collapse, nature of attributes, genus worries) in light of this framework, before concluding with philosophical payoffs.

Principle 1: Pure Actuality — No Potentiality

The classical argument begins with Aristotle’s analysis of motion and causation. From Physics VIII he argues that whatever is in motion is moved by something; an infinite regress of moved movers would explain nothing; therefore one arrives at a first mover that is not itself moved by anything else. Aristotle states the key step succinctly: “there must be some first movent that is not itself moved by anything else” (Aristotle 1930, Physics VIII.5). This is not a fideistic assertion but a causal inference: if the explanatory chain of movers had no unmoved first term, the actuality required to account for motion would never be secured. The result is not merely an “unmoved” but a mover whose mode of being must fit the work it does. An unmoved mover cannot actualize by being actualized, so it must be actual in itself.

Aristotle makes that metaphysical point explicit in Metaphysics Λ. He argues from the eternity and continuity of cosmic motion to a principle whose being is actuality through and through. The crucial conclusion appears in his compressed summary: “There must, then, be such a principle, whose very essence is actuality” (Aristotle 1933, Metaphysics XII.6, 1071b17–20). This is not a change of subject from physics to theology; it is an ontological deepening of the causal thesis. If the ultimate source of motion had even a shred of unrealized potency, some further actualizer would be needed to bring that potency to act. The regress would resume. Hence Aristotle immediately characterizes the first principle in four coordinated notes: “there is something which moves without being moved, being eternal, substance, and actuality” (Aristotle 1933, Metaphysics XII.7, 1072a24–26). “Eternal” rules out perishing and becoming, “substance” names what exists in itself, and “actuality” excludes the composition of act with any unrealized potentiality. The first principle is actuality as such.

Aristotle also makes an explicit priority claim. He warns that to make potency prior in reality is a mistake: “To suppose potency prior to actuality, then, is in a sense right, and in a sense not” (Aristotle 1933, Metaphysics IX.8, 1050a6–7). Potency is prior in our analysis and in time for sublunary changes, but not in substance or causality at the top of the explanatory order. That is why he pairs unmovedness with actuality again when driving the point home: “Since there is something which moves while itself unmoved, existing actually, this can in no way be otherwise” (Aristotle 1933, Metaphysics XII.7, 1072a27–30). In other words, if the first mover were not already actual, it could not ground the ceaseless actuality of cosmic motion. Aristotle’s discussion continues by noting that such a first mover “exists of necessity” and that this necessity is good and without matter or change; for such a being, actuality is not an added feature but its very way of being (see Aristotle 1933, Metaphysics XII.7, 1072b3–10).

Once pure actuality is in view, two further conclusions follow that are vital for the “no-composition” and “unifier” tests this paper employs. First, pure actuality cannot include unrealized powers standing in potency to completion, because that would invite composition by act and potency within the first principle. Second, if the first principle were composed of distinct parts or distinct operative principles, each part would either be in potency relative to the whole (needing completion by others) or would require a cause of unity beyond itself. Either way, the whole would be dependent and thus not ultimate. Aristotle himself gestures here by insisting that the first mover is without magnitude and indivisible precisely because it is unmoved actuality; a divisible composite would harbor potentials and be moveable (see Aristotle 1933, Metaphysics XII.6–7, 1071b6–12; 1073a5–10).

Aquinas takes Aristotle’s result and makes the anti-composition consequence explicit. In Summa theologiae I, q.3, a.7 he argues that composition is incompatible with ultimacy for several reasons, two of which directly bear on act and potency. First, “every composite is posterior to its component parts, and is dependent on them.” Second, “every composite has a cause, for things in themselves different cannot unite unless something causes them to unite.” Both conclusions are marshaled in service of divine simplicity; both would be violated if the first principle had really distinct operative parts (Aquinas 1947–48, ST I, q.3, a.7, respondeo). This is not a mere scholastic slogan. Aquinas is drawing out what it means for the first principle to be pure actuality. If the first principle contained act together with unrealized potency, or if it required a unifier to bind distinct constituents, it would depend on either an intrinsic principle of composition or an extrinsic cause of unity. Either option contradicts the role of a first cause.
This is why the classical tradition insists that the relevant “parts” cannot obtain at the ultimate level. To suppose, for example, multiple complete and distinct operative principles in the first cause is to reintroduce potency and dependence in two ways. Each operative principle, if really distinct, either limits or completes the others, which signals potency. And the unity of the subject across these principles would either require a further unifier, which is a cause, or else would reduce to identity of operation, which collapses the supposed plurality. Aristotle’s “actuality” language blocks the first horn; Aquinas’s “every composite has a cause” blocks the second. The upshot is that the first principle cannot be a heap of perfections, powers, or faculties whose unity is subsequent to that plurality. If it were, it would not be first.

The purity claim also clarifies why the first principle cannot be measured by a common scale. Anything that falls under a genus is composite of genus and differentia, which entails potency to the generic nature actualized by the specific difference. Aristotle’s analysis that the first principle is actuality “and not otherwise” forbids that genus-difference composition at the top (see Aristotle 1933, Metaphysics XII.7, 1072a27–30). Aquinas draws the inference: if God were in a genus, something more general would be prior to him; therefore the first principle is not in a genus (Aquinas 1947–48, ST I, q.3, a.5). Although that is a distinct argument, it is parasitic on the same act–potency analysis that yields pure actuality and therefore bears directly on why the “no-common-measure” test is a consequence of the purity claim.
To summarize the argumentative structure: from the fact of motion and the impossibility of an infinite regress of moved movers, Aristotle infers a first unmoved mover. From the causal role required of that mover, he infers that its “very essence is actuality” and repeatedly insists on unmovedness, eternity, and actuality as co-instantiated notes (Aristotle 1933, Metaphysics XII.6–7, 1071b17–20; 1072a24–30). From pure actuality, it follows that the first principle cannot harbor unrealized potencies or really distinct operative constituents, since those would require composition and a cause of unity. Aquinas states the compositional consequence in general form—“every composite has a cause”—as a reason why the first being must be absolutely simple (Aquinas 1947–48, ST I, q.3, a.7). The conclusion is philosophical and deductive: the first principle is pure act, and therefore no account that builds it out of distinct operative parts or properties can succeed without either abandoning ultimacy or collapsing the distinctions into identity of operation.

Quoted passages used above, with full inline citations

“there must be some first movent that is not itself moved by anything else” (Aristotle 1930, Physics VIII.5).
“There must, then, be such a principle, whose very essence is actuality” (Aristotle 1933, Metaphysics XII.6, 1071b17–20).
“there is something which moves without being moved, being eternal, substance, and actuality” (Aristotle 1933, Metaphysics XII.7, 1072a24–26).
“To suppose potency prior to actuality, then, is in a sense right, and in a sense not” (Aristotle 1933, Metaphysics IX.8, 1050a6–7).
“Since there is something which moves while itself unmoved, existing actually, this can in no way be otherwise” (Aristotle 1933, Metaphysics XII.7, 1072a27–30).
“The first mover, then, exists of necessity” (Aristotle 1933, Metaphysics XII.7, 1072b3–4).
“every composite is posterior to its component parts, and is dependent on them” (Aquinas 1947–48, Summa Theologiae I, q.3, a.7, respondeo).
“every composite has a cause, for things in themselves different cannot unite unless something causes them to unite” (Aquinas 1947–48, Summa Theologiae I, q.3, a.7, respondeo).
Bibliography (Chicago author–date, print-style; no links)
Aristotle. 1930. Physics. Translated by R. P. Hardie and R. K. Gaye. In The Works of Aristotle, edited by W. D. Ross. Oxford: Clarendon Press. Book VIII.
Aristotle. 1933. Metaphysics. Translated by W. D. Ross. Oxford: Clarendon Press. Books IX and XII.
Aquinas, Thomas. 1947–48. Summa Theologiae. Translated by the Fathers of the English Dominican Province. 3 vols. New York: Benziger Brothers. I, q.3, aa.5 and 7.

Principle 2: Cause of Unity — Why Any Real Many Is One Only Through a Unifier

Aristotle begins not with a brute stipulation but with a problem. When a whole is genuinely one rather than a mere heap, what makes the many to be one? He formulates the question and the thesis together: “To return to the difficulty which has been stated with respect both to definitions and to numbers, what is the cause of their unity? In the case of all things which have several parts and in which the totality is not, as it were, a mere heap, but the whole is something beside the parts, there is a cause; for even in bodies contact is the cause of unity in some cases, and in others viscosity or some other such quality” (Aristotle 1933, Metaphysics VIII.6, 1045a8–12, trans. W. D. Ross). The scope is deliberately general. Aristotle speaks of definitions and numbers alongside bodies to show that the need for a cause of unity is not confined to physics but belongs to first philosophy. The illustrations of contact and viscosity are not the point, they are reminders that even in the most concrete cases the “one” is explained by something over and above a bare enumeration of parts. The whole is “something beside the parts,” which means that unity, where it is real, has an explanation.

Aristotle does not leave the point at the level of diagnosis. He immediately sketches the pattern of explanation by appeal to formal and efficient causes. After distinguishing what is potentially from what is actually, he resolves the puzzle by assigning to form the work of making the many to be one, and to an agent the work of bringing a potential unity into act: “The difficulty will no longer be thought a difficulty if one element is matter and another is form, and one potentially and the other actually. What, then, causes this—that which was potentially to be actually—except, in the case of things which are generated, the agent?” (Aristotle 1933, Metaphysics VIII.6, 1045a33–b7, trans. W. D. Ross). The pattern is precise. In substances the form is the principle that makes this matter to be one thing rather than a heap. Where generation is involved, an efficient cause is required to reduce the potency to act. For things that are not generated but are intelligible unities, Aristotle adds that each “is by its nature essentially a kind of unity,” which is to say that wherever composition is denied, there is no need to seek a further cause of unity because the very being at issue is already one by nature. In every other case, where a plurality forms a single whole, “there is a cause” of the unity (Aristotle 1933, Metaphysics VIII.6, 1045a8–12).

From this base the classical tradition draws a general metaphysical axiom: wherever a whole is truly one and not a heap, its unity is explained by a unifying principle. That principle need not be an extra part in addition to the many. In hylomorphic cases it is the form; in aggregates it may be contact, viscosity, or order; in generated substances it includes an agent cause that effects the passage from potency to act. The common thread is that the parts, as parts, are prior in analysis, while the one whole, as one, exhibits a note that is not secured by mere addition. Aristotle’s contrast between heaps and definitions makes the point vivid. A poem can be “one” as a connected series of words, but the unity of a definition is the unity of form; only the latter gives a reason why the many is one other than aggregation.

Aquinas turns Aristotle’s insight into a general principle and then applies it to the question of the first principle. In Summa theologiae I, q.3, a.7 he argues that the first being cannot be composite, and two of his reasons directly formalize Aristotle’s claim. First, “every composite is posterior to its component parts, and is dependent on them.” Second, “every composite has a cause, for things in themselves different cannot unite unless something causes them to unite” (Aquinas 1947–48, Summa Theologiae I, q.3, a.7, respondeo, trans. Fathers of the English Dominican Province). The first sentence states the ontological priority of parts to whole wherever composition obtains, which already signals non-ultimacy. The second sentence restates, in a maximally general way, the very principle Aristotle articulated in Metaphysics VIII.6: if there is a genuine plurality that is one, the unity is because of a cause. In Aquinas’s context, this principle serves a decisive function. If the first principle were composite, it would require a cause of unity and would therefore not be first. Since the first principle is uncaused, it cannot be composite.

This cause-of-unity axiom bears directly on models of deity that posit a plurality of complete operative principles within the one subject. If, for example, one proposes that the divine subject contains several complete sets of rational cognitive faculties, each sufficient for personal operation, then by Aristotle’s standard the plurality could be one only because of a unifier. On Aquinas’s formulation, such a unity would be caused, which contradicts the ultimacy required of the first principle. If one insists the unifier is identical with the subject yet remains really distinct from the operative faculties, composition persists and the same problem returns. If one denies any distinct unifier, the many remains many and the proposed subject is not one substance. The only way to avoid these horns while retaining ultimacy is to deny the real plurality at the root and affirm identity of operation in the subject, which is the classical simplicity thesis.

The principle also clarifies why appeals to relational harmony do not solve the metaphysical problem. Unity of love, unity of will, or mutual indwelling can be real and perfect relations. As relations, however, they presuppose termini already in act. They do not supply, by themselves, the principle by which several really distinct operative sources belong to one substance without composition. Aristotle’s very examples are instructive. When we explain why a clump of matter is one body rather than a heap, we do not say “it is many but behaves harmoniously.” We name the cause of unity: contact, viscosity, or most fundamentally the form that makes it one in act. The explanatory demand does not disappear at higher levels of analysis. If anything, it tightens, because the more fundamental the unity, the less acceptable it is to posit a cause of unity distinct from what is first.

In this way the cause-of-unity principle anchors a neutrality-preserving test for ultimacy. A candidate first principle that includes a real plurality of complete operative sources either depends on a unifying cause, which forfeits ultimacy, or collapses the plurality by identifying the operations in a single simple act. The first option contradicts the status of the first cause as uncaused. The second option is the identity thesis that classical simplicity articulates. Aristotle’s question, “what is the cause of their unity?” together with Aquinas’s axiom, “every composite has a cause,” leave no third way for a genuinely composite ultimate.

Quoted passages used above, with full inline citations

“What is the cause of their unity? In the case of all things which have several parts and in which the totality is not, as it were, a mere heap, but the whole is something beside the parts, there is a cause; for even in bodies contact is the cause of unity in some cases, and in others viscosity or some other such quality” (Aristotle 1933, Metaphysics VIII.6, 1045a8–12, trans. W. D. Ross).
“The difficulty will no longer be thought a difficulty if one element is matter and another is form, and one potentially and the other actually. What, then, causes this—that which was potentially to be actually—except, in the case of things which are generated, the agent?” (Aristotle 1933, Metaphysics VIII.6, 1045a33–b7, trans. W. D. Ross).
“Every composite is posterior to its component parts, and is dependent on them. Thirdly, because every composite has a cause, for things in themselves different cannot unite unless something causes them to unite” (Aquinas 1947–48, Summa Theologiae I, q.3, a.7, respondeo, trans. Fathers of the English Dominican Province).
Bibliography (Chicago author–date, print-style; no links)
Aristotle. 1933. Metaphysics. Translated by W. D. Ross. Oxford: Clarendon Press. Book VIII, chapter 6 (Bekker 1045a8–12; 1045a33–b7).
Aquinas, Thomas. 1947–48. Summa Theologiae. Translated by the Fathers of the English Dominican Province. 3 vols. New York: Benziger Brothers. I, q.3, a.7.

Principle 3: Simplicity from Non-Posteriority and Non-Causedness

Aquinas’s simplicity proof is not an isolated thesis but the point where the classical account of unity and causality becomes categorical. He argues that what is first in the order of explanation cannot be composite, because composition necessarily brings with it both ontological posteriority to parts and dependence on a cause of unity. The formal structure is concentrated in Summa theologiae I, q.3, a.7, where he writes in the respondeo: “every composite is posterior to its component parts, and is dependent on them. … every composite has a cause, for things in themselves different cannot unite unless something causes them to unite.” (Aquinas 1947–48, ST I, q.3, a.7, respondeo.) (New Advent) The syllogism Aquinas means to license is straightforward. All composites are caused and posterior to their parts; the first principle is uncaused and prior to all; therefore the first principle is not composite. The transition from those general premises to the conclusion about God is immediate once “first principle” has been established in prior questions as the uncaused cause. If the first cause were composite, it would fall under the general rule that composites require a composer or unifier; it would therefore be an effect. That is a contradiction. Hence the first cause must be without composition.

Aquinas’s conclusion in the same question is unambiguous. After surveying and excluding the principal seats of composition, he closes the article with the definitive line: “it is clear that God is nowise composite, but is altogether simple.” (Aquinas 1947–48, ST I, q.3, a.7, respondeo.) (New Advent) The adverb “altogether” performs crucial work, because it signals that Aquinas is not merely denying quantitative parts, but any real composition whatever. That universality is not a rhetorical flourish. In the respondeo he explicitly ties the result “from the previous articles,” which have just ruled out, one by one, the principal modes of composition that would introduce dependence or potency into the first principle.

The survey of those modes can be sketched briefly from the same question. First, there is composition of matter and form. Aquinas denies this of God: “It is impossible that matter should exist in God,” because whatever is in potency to form is not pure act; but what is first must be act without potency. (Aquinas 1947–48, ST I, q.3, a.2, respondeo.) (New Advent) Second, there is composition of quiddity (essence) and subject; Aquinas rejects this by insisting that God is not one thing having a nature other than Himself, but is identical with His own essence. (Aquinas 1947–48, ST I, q.3, a.3.) (New Advent) Third, and pivotally, there is composition of essence and existence. Here Aquinas argues in a manner that reveals the deep link to act and potency: “God is not only His own essence … but also His own existence,” since any thing whose existence is other than its essence either receives that existence from constituents or from an exterior cause; neither is admissible in the first cause. (Aquinas 1947–48, ST I, q.3, a.4, respondeo.) (New Advent) Fourth, Aquinas denies composition by genus and difference, since to be a species in a genus implies a formal composition that imports potency relative to the generic nature, but in God there is no potency. (Aquinas 1947–48, ST I, q.3, a.5.) (New Advent) Fifth, he excludes subject and accident, because accidents bespeak actuality supervening on a subject capable of receiving them, which would reintroduce potency and dependence. (Aquinas 1947–48, ST I, q.3, a.6.) (New Advent) The cumulative upshot of these exclusions is summarized in the claim already quoted: “God is nowise composite, but is altogether simple.” (Aquinas 1947–48, ST I, q.3, a.7.) (New Advent)

Why does the argument compel, rather than merely recommend, simplicity? Aquinas’s two axioms in a.7 do the decisive work. The first axiom, that “every composite is posterior to its component parts, and is dependent on them,” states an ontological priority: the whole, as composite, does not have its being underived, but stands to its constituents as dependent to those prior principles. The second axiom, that “every composite has a cause,” translates Aristotle’s more general insight about unity into the idiom of efficient causality. Where there is a plurality that is truly one whole and not a heap, “things in themselves different cannot unite unless something causes them to unite.” (Aquinas 1947–48, ST I, q.3, a.7, respondeo.) (New Advent) The first axiom blocks ultimacy by part-priority; the second blocks it by causal priority. In both cases, to ascribe composition to the first cause is to ascribe to it a dependence that is incompatible with its role as first.

The supporting articles show how each familiar kind of composition would entail precisely such dependence. Matter–form composition would place God under the regime of potency and act, with form actualizing matter; but the first principle, as pure act, admits no unrealized potency. (Aquinas 1947–48, ST I, q.3, a.2.) (New Advent) Composition of quiddity and subject would make God to be a bearer of a nature, rather than subsistent nature itself; that entails a real distinction within God between what He is and that by which He is, which would fracture ultimacy. (Aquinas 1947–48, ST I, q.3, a.3.) (New Advent) Composition of essence and existence would demote God to a recipient of existence, either from principles intrinsic to a quiddity or from an exterior cause. Aquinas’s argument there is explicit: if existence is added to an essence, it is received; what is received is caused; but the first cause is uncaused; therefore in God essence and existence are identical. (Aquinas 1947–48, ST I, q.3, a.4, respondeo.) (New Advent) Composition by genus and difference would place God under a common measure more general than Himself, a point Aquinas develops by appeal to Aristotle’s denial that “being” is a genus; genus–difference composition therefore smuggles in potency and subordination. (Aquinas 1947–48, ST I, q.3, a.5.) (New Advent) Finally, subject–accident composition would make God a subject capable of receiving perfections that are not identical with His essence, which again implies potency relative to those perfections. (Aquinas 1947–48, ST I, q.3, a.6.) (New Advent)
With those possibilities eliminated, a.7 binds the results together under a general principle: if anything in God were a composite, God would be posterior to parts and caused by a composer; but the first principle is by definition uncaused and prior; therefore He is not composite. The consequentia is not merely logical but metaphysical. Posteriority to parts marks an ontological order in which the composite stands as dependent upon its constituents, while the appeal to a cause of unity marks an efficient order in which the composite stands as an effect of a unifier. Aquinas’s phrase “things in themselves different cannot unite unless something causes them to unite” makes clear that he is not speaking only of physical aggregation or of artificial artifacts, but of composition as such. (Aquinas 1947–48, ST I, q.3, a.7, respondeo.) (New Advent) In the case of the first principle, any such appeal to a cause would exceed the first, which is impossible by hypothesis.

Two clarifications strengthen the dialectical force of the proof. First, simplicity here is not the claim that God is a thin abstraction; it is the denial that the first principle’s being is the resultant of a prior plurality. When Aquinas says in a.4 that “God is not only His own essence … but also His own existence,” he is excluding the very composition that would make God stand to existence as recipient to donor. (Aquinas 1947–48, ST I, q.3, a.4.) (New Advent) Second, the simplicity conclusion is the logical terminus of the act–potency analysis that already grounds the first principle’s pure actuality. If there were composition in God, there would be a respect in which He is in potency to the unity or actuality of the whole, or a respect in which different principles must be made one. Either scenario contradicts pure act and ultimacy. Hence Aquinas can state without qualification, on the basis of the foregoing, that “God is nowise composite, but is altogether simple.” (Aquinas 1947–48, ST I, q.3, a.7.) (New Advent)
Bibliography (Chicago author–date; print-style; no hyperlinks)
Aquinas, Thomas. 1947–48. Summa Theologiae. Translated by the Fathers of the English Dominican Province. 3 vols. New York: Benziger Brothers. I, q.3, aa.2–7.

Principle 4: Not in a Genus – No Common Measure

Aquinas frames the issue with uncommon directness: “it is clear that God is not in a genus” (Aquinas 1947–48, ST I, q.3, a.5, respondeo). He offers this not as a mere theological assertion but as the metaphysical upshot of how genera and species account for definitional unity. To be in a genus is to be constituted by genus and difference, so that what the thing is results from a formal structure more general than the thing itself. If anything were the first principle, nothing more general could be prior to it, whether in reality or in thought. Therefore, if the first principle were in a genus, it would be posterior in account to a more general ratio that measures and limits it, which contradicts ultimacy. Aquinas underlines this with a sed contra that states the point in logical form: “In the mind, genus is prior to what it contains. But nothing is prior to God either really or mentally. Therefore God is not in any genus” (Aquinas 1947–48, ST I, q.3, a.5, sed contra). He then proceeds in the body of the article to eliminate the only two ways a thing could be “in” a genus. There is the way of species under genus, which would force God to be composed of genus and difference and so introduce a real composition of potentiality and actuality into the first principle; and there is the way of reduction, as when principles or privations are “reducible” to a genus as its foundations, which would limit the first principle’s scope to a particular order. In the first case, genus–difference structure imports composition; in the second, reducibility would deny the first principle’s causal primacy across the whole order of beings. Since both routes are incompatible with ultimacy, neither is available (Aquinas 1947–48, ST I, q.3, a.5, respondeo).

Aristotle supplies the anchor premise that makes this exclusion transparent. In analyzing the logical form of predication across the categories, he denies that “being” (to on) or “one” (hen) can function as a genus. The classic formulation in the Ross translation reads: “It is not possible that either unity or being should be a single genus … for the differentiae of any genus must each of them both have being and be one” (Aristotle 1933, Metaphysics III.3, 998b22–27). The argument is compact. If “being” were a genus, its differentiae would have to be outside the genus in order to divide it. But there is nothing outside being that could serve as a genuine differentia; non-being cannot exist to do the dividing. The same line applies to unity: any differentia would itself be one, so it could not stand outside unity to divide it. Thus neither being nor unity is a genus. Aquinas explicitly invokes this Aristotelian point in the course of the article: “the Philosopher has shown (Metaph. iii) that being cannot be a genus” (Aquinas 1947–48, ST I, q.3, a.5, respondeo). The consequence for the first principle is immediate. If God’s essence is identical with existence and if “being” cannot be a genus, then there is no possible genus under which God could fall; and if there were any other putative genus, it would either be more general than being, which is incoherent, or it would reduce to being under some aspect, which returns us to the same contradiction.

Aquinas’s own reasoning makes the act–potency mechanics of genus–difference explicit. A species is constituted by adding a difference to a genus, and “that from which the difference is derived is always related to that which is limited by the difference, as actuality is related to potentiality” (Aquinas 1947–48, ST I, q.3, a.5, respondeo). The genus contributes what is common and indeterminate; the specific difference actualizes that determinable content into a determinate species-nature. In any such constitution there is a real composition corresponding to the logical composition: genus is in potency relative to difference, difference is in act relative to genus. If that kind of structure were found in the first principle, then there would be a real composition of potentiality and actuality in what must, by Principle 1, be pure actuality. The no-genus conclusion is therefore not an isolated thesis but rather the logical face of the pure-act result. The first principle cannot be “a kind of F,” since kinds are internally stratified by the very composition we have already excluded.

Two further consequences seal the argument and motivate the “no common measure” corollary. First, anything in a genus is comparable with other species under that genus according to a common ratio. Species share a common measure precisely by being measured under a more general account. If the first principle were similarly comparable under a common measure, then it would be limited and rankable alongside other instances that share the genus. This is what Aquinas rejects when he meets the objection that God must be a measure for substances. He replies that the only sense in which God is “measure” is not proportionate or commensurate, but exemplar: “This objection turns upon proportionate measure … Now, God is not a measure proportionate to anything” (Aquinas 1947–48, ST I, q.3, a.5, ad 2). Things resemble God and so have being by participation, but there is no genus-like yardstick common to God and creatures that would rank them on one scale. Second, because genus–difference composition is one member of the more general family of compositions excluded in ST I, q.3, a.7, the non-genus status of the first principle contributes directly to simplicity: where there is no genus, there is no differentiating constituent added to a common nature, and thus no room for the kind of real internal plurality that would require a cause of unity (Aquinas 1947–48, ST I, q.3, a.7, respondeo).

The upshot for classical theism is both conceptual and ontological. Conceptually, it denies that there is any definitional path to God that proceeds by taking a common kind and adding a differentia. Definitions of the genus–differentia sort presuppose a broader category under which the definiendum falls. Since nothing is broader than the first principle, such a path is closed. Ontologically, it denies that there is any common scale for co-ranking God and creatures. If one attempted to measure divine perfections and creaturely perfections by a single metric—say, “power” univocally construed—one would be positing, in effect, a genus of “power” within which God is a species or an instance alongside others. The Aristotelian–Thomistic prohibition on being a genus is the strict barrier against that move. Either the purported scale is extrinsic to the first principle, in which case the first principle is subordinated to a measure more general than itself, or the “scale” is nothing but the first principle as exemplar, in which case it ceases to be common. In neither case is there a common measure that treats God and creatures as co-instantiating a genus. This is exactly why the non-genus thesis is essential for the Aggregation test: without it, efforts to construct “maximal greatness” by aggregating independently defined properties smuggle in a higher yardstick that would limit the first principle.

The dialectical structure can be stated with precision. If a thing is in a genus, then its essence is constituted by genus and differentia; if its essence is so constituted, then there is a composition of potency and act at the level of essence; if there is such composition, the thing depends on a cause of unity and is posterior to its constituents. But the first principle is neither posterior nor caused. Therefore it is not in a genus. As Aquinas summarizes: “it is clear that God is not in a genus” (Aquinas 1947–48, ST I, q.3, a.5, respondeo). The Aristotelian premise that “being” is not a genus closes off what might otherwise have seemed the only candidate genus, given that the first principle’s essence is existence. What remains is a non-generic, non-measurable source, which others can resemble only by participation, never by co-membership under a higher kind.

Works Cited (Chicago author–date; print-style)
Aristotle. 1933. Metaphysics. Translated by W. D. Ross. Oxford: Clarendon Press. Book III, chapter 3 (Bekker 998b22–27).
Aquinas, Thomas. 1947–48. Summa Theologiae. Translated by the Fathers of the English Dominican Province. 3 vols. New York: Benziger Brothers. I, q.3, a.5 (sed contra; respondeo; ad 2); I, q.3, a.7 (respondeo).

Principle 5: Avicenna’s Necessary Existent – Pure Existence, No Quiddity

Avicenna’s analysis of the Wājib al-Wujūd (Necessary Existent) gives a stringent metaphysical test for ultimacy that converges with, and sharpens, the Aristotelian–Thomistic results already stated. The core claim is not a slogan about perfection, but a worked thesis about the constitution of the First: the Necessary Existent has no quiddity (māhiyya) other than its very being. In Michael E. Marmura’s standard English translation of the Ilāhiyyāt of the Healing, Avicenna states the conclusion in unambiguous terms: “The First has no quiddity other than His individual existence.” (Avicenna 2005, Ilāhiyyāt VIII.4, 274). He restates the point in a way that makes the modal structure plain: “there is no quiddity for the Necessary Existent other than its being the Necessary Existent. And this is [the thing’s] ‘thatness’ [inniyya/anniyya].” (Avicenna 2005, Ilāhiyyāt VIII.4, 276–277). The same chapter sequence underlines that this “thatness” excludes any additional differentiating component: “necessary existence has no quiddity that connects with it other than necessary existence.” (Avicenna 2005, Ilāhiyyāt VIII.5, 279). Avicenna also gives the closely related formulation that ties the First’s priority to its noncomposite status and to the dependence of everything else: “The First, hence, has no quiddity. Those things possessing quiddities have existence emanate on them from Him. He is ‘pure existence’ [mujarrad al-wujūd] with the condition of negating privation and all other description of Him… [E]verything other than Him has addition [ziyāda].” (Avicenna 2005, Ilāhiyyāt VIII.4, 273–274).

These lines do several kinds of philosophical work at once. First, they deny essence–existence composition at the first principle. For Avicenna, a quiddity is what answers the “what is it?” question, and as such is in principle shareable, definable, and combinable with differentiae. Where there is a quiddity that is anything other than “to be the Necessary Existent,” existence is “added” to that quiddity; what has quiddity in that sense is a recipient of being. By contrast, “The First has no quiddity other than His individual existence” (Avicenna 2005, Ilāhiyyāt VIII.4, 274). If the First had a quiddity really distinct from its existence, it would fall under the structure that characterizes possibles: it would stand to existence as to something received, or at best derivable; but Avicenna has just argued that “those things possessing quiddities have existence emanate on them from Him” (Avicenna 2005, Ilāhiyyāt VIII.4, 273–274). The First, therefore, cannot have a distinct quiddity without ceasing to be first.

Second, the quoted claims entail that the First is not in a genus. Genus and differentia are by definition constituents of a quiddity; if “there is no quiddity for the Necessary Existent other than its being the Necessary Existent” (Avicenna 2005, Ilāhiyyāt VIII.4, 276–277), then there is nothing for a genus–differentia analysis to seize upon. To place the First under a genus would be to assign it a common whatness and then mark it off by a differentia that adds to that common nature. Avicenna denies precisely this: for the First there is no such common whatness other than necessary existence itself. That is why the follow-up line makes the denial general: “necessary existence has no quiddity that connects with it other than necessary existence” (Avicenna 2005, Ilāhiyyāt VIII.5, 279). The First cannot be an instance under a more general concept without importing quiddity as an added constituent; the price of genus membership would be loss of ultimacy.

Third, Avicenna’s words exclude internal composition of the First by any plurality of constituents, whether they be “parts,” properties, or really distinct operative principles. His use of mujarrad al-wujūd (“pure existence”) is exact: the First is existence stripped of privation and alien description; “everything other than Him has addition” (Avicenna 2005, Ilāhiyyāt VIII.4, 273–274). An “addition” is precisely what would be required for a cluster of distinct features or faculties to stand together in one subject; but the First is defined by the absence of any such addition. Here Avicenna’s framework dovetails with the Aristotelian requirement that a many can be one only by a cause of unity, and with Aquinas’s axiom that “every composite has a cause” and is “posterior to its component parts” (Aquinas 1947–48, ST I, q.3, a.7, respondeo). In Avicenna’s idiom, whatever “has addition” is not the First but derives from the First; whatever would require a unifier lies on the side of the caused, not on the side of the uncaused giver of being.

Fourth, these passages supply a modal reason for the First’s absolute simplicity. If the First had a quiddity other than necessary existence, the First would be a being possible in itself and necessary through another, because a quiddity considered as such does not entail existence. Avicenna forestalls that possibility by identifying the “thatness” of the First with necessary existence itself: “there is no quiddity for the Necessary Existent other than its being the Necessary Existent” (Avicenna 2005, Ilāhiyyāt VIII.4, 276–277). Thus the First is necessary in itself not by addition of a necessity-conferring property to an essence, but by being identical with necessary existence. Composition of essence and existence is the modal signature of what is not first.

The inferential structure that supports these conclusions is straightforward and can be set out without suppressing any controversial premise. Begin with Avicenna’s uncontentious observation that for possible things there is a real distinction between what they are and that they are. In these, quiddity explains the possibility of existence, not its actuality; “those things possessing quiddities have existence emanate on them from Him” (Avicenna 2005, Ilāhiyyāt VIII.4, 273–274). If one tried to ascribe a similar structure to the First, making it a case of “quiddity plus existence,” one would thereby relocate the First into the class of those things that receive existence, which contradicts its status as the source from whom they receive. The only escape from that contradiction is to deny that the First has any such quiddity and to affirm that for the First, being and “thatness” are identical: hence, “The First has no quiddity other than His individual existence” (Avicenna 2005, Ilāhiyyāt VIII.4, 274), and “necessary existence has no quiddity that connects with it other than necessary existence” (Avicenna 2005, Ilāhiyyāt VIII.5, 279). Once that identification is in place, genus membership is excluded by form of speech, because genus–differentia definitions presuppose a quiddity that is not identical with being. So too is any inner plurality of constituents excluded, because any such plurality would be an “addition” and so a mark of derivation rather than of ultimacy. The First is mujarrad al-wujūd, pure existence without added quiddity.

This Avicennian result carries the same practical upshot for classical theism as the Aristotelian–Thomistic principles. It blocks the “attribute-cluster” approach to ultimacy at the root. If one describes the ultimate as a bearer of many really distinct great-making features, one has proposed a subject that “has addition.” That is, one has posited a structure in which being belongs by way of added components, whether these be thought of as essential properties or as complete operative faculties. Avicenna’s First excludes such addition by definition; his repeated “no quiddity other than necessary existence” removes the space for a metaphysical cluster to be co-instantiated in the First. Conversely, if one tries to avoid addition by collapsing the distinct features into one, one returns to the very identity of being and perfection that Avicenna states without qualification in the lines quoted. Either way, the Necessary Existent is not a composite instance measured by a common scale; it is the simple giver of existence, not a case of “essence plus existence.”

In sum, Principle 5 is Avicenna’s supervised identity claim: the First’s “what-it-is” is nothing other than necessary existence; therefore it is not composed of essence and existence, has no genus or differentia, and does not harbor any plurality that would require a cause of unity. The First is mujarrad al-wujūd, pure existence without addition; “The First has no quiddity other than His individual existence” (Avicenna 2005, Ilāhiyyāt VIII.4, 274), and “necessary existence has no quiddity that connects with it other than necessary existence” (Avicenna 2005, Ilāhiyyāt VIII.5, 279). Everything else, precisely insofar as it has quiddity, “has addition” and receives existence from Him (Avicenna 2005, Ilāhiyyāt VIII.4, 273–274). That is why Avicenna’s treatment decisively closes the door on genus membership and internal aggregation at the level of the first principle.

Bibliography (Chicago author–date; print-style; no hyperlinks)
Avicenna. 2005. The Metaphysics of the Healing (al-Ilāhiyyāt). Translated by Michael E. Marmura. Provo, UT: Brigham Young University Press. Book VIII, chapters 4–5, pp. 273–279.
Aquinas, Thomas. 1947–48. Summa Theologiae. Translated by the Fathers of the English Dominican Province. 3 vols. New York: Benziger Brothers. I, q.3, a.7 (respondeo).

Principle 6: Maimonides on Attributes and Unity

Maimonides frames the thesis with unusual bluntness in the opening chapter of his sustained treatment of divine attributes. After expelling corporeality from any adequate conception of God, he draws the immediate corollary that so-called “essential attributes” must go as well. He writes: “you must understand that God has no essential attribute in any form or in any sense whatever, and that the rejection of corporeality implies the rejection of essential attributes. Those who believe that God is One, and that He has many attributes, declare the unity with their lips, and assume plurality in their thoughts.” (Maimonides 1904, Guide for the Perplexed I.50, p. 66–67). This is not a mere terminological caution. It is a metaphysical claim about what would follow if we multiplied perfections in God as really distinct positives. If we treat “omniscience,” “omnipotence,” and the like as additional constituents, we no longer have an unqualified unity at the level of what there is, but a composite whose putative oneness would have to be explained by appeal to a cause of unity. That move is excluded for the first principle by the classical demand that the ultimate be uncaused and posterior to nothing. (Internet Sacred Text Archive)

The backbone of Maimonides’ argument appears in the next chapter, where he states the governing metaphysical rule about attributes as such. He insists that an “attribute” is by nature something added-on rather than identical with the essence of a thing; as such it is accidental in the strict sense and signals composition. His language is explicit: “For it is a self-evident truth that the attribute is not inherent in the object to which it is ascribed, but it is superadded to its essence, and is consequently an accident; … he who merely rejects the appellation ‘accidents’ in reference to the attributes of God, does not thereby alter their character; for everything superadded to the essence of an object joins it without forming part of its essential properties, and that constitutes an accident. Add to this the logical consequence of admitting many attributes, viz., the existence of many eternal beings.” (Maimonides 1904, Guide I.51, p. 69). The argument turns on two connected inferences. First, if a predicate is truly an “attribute,” it is not identical with what the thing is; therefore its presence marks composition and so dependence upon either the attribute as a distinct constituent or upon a unifier that binds attribute to subject. Second, if the divine attributes are posited as really distinct and eternal, one has as many uncaused “beings” as there are such attributes—an outcome that destroys divine unity in principle rather than in name. (Internet Sacred Text Archive)

From these premises he articulates the positive unity claim that carries the weight of the whole program: “There cannot be any belief in the unity of God except by admitting that He is one simple substance, without any composition or plurality of elements: one from whatever side you view it, and by whatever test you examine it: not divisible into two parts in any way and by any cause, nor capable of any form of plurality either objectively or subjectively.” (Maimonides 1904, Guide I.51, p. 69). Two features matter for classical theism. “Simple substance” here is not the minimal scholastic notion of an immaterial subject that still bears really distinct accidents; it is the maximal thesis that in God there is no composition by any principle—no parts, elements, accidents, differentiating features, or really distinct positives at all. And “by any cause” closes the door on saving unity by appeal to a binder. If unity must be caused, the subject is no longer ultimate. (Internet Sacred Text Archive)

The methodological corollary follows in the same sequence of chapters. Because positive essential predications threaten to multiply what is in God, the only “true” attributions are negative in form, or else they function as denials of creaturely defect. “The negative attributes of God are the true attributes: they do not include any incorrect notions or any deficiency whatever in reference to God, while positive attributes imply polytheism, and are inadequate, as we have already shown.” (Maimonides 1904, Guide I.58, p. 82). The via negativa is therefore not rhetorical modesty but a principled device that preserves unity while still allowing us to speak without falling into silence. On this method “wise,” said of God, does not add an item alongside the essence; rather it denies ignorance and affirms the absence of defect that we name “wisdom” in a way proportioned to creaturely minds. (Internet Sacred Text Archive)

Placed within the wider classical framework, Maimonides’ strictures integrate directly with the two earlier principles that do the heavy lifting against composition. If “every composite is posterior to its component parts, and is dependent on them … every composite has a cause,” then a being that is first in the relevant order cannot be composite in any respect (Aquinas 1947–48, Summa Theologiae I, q.3, a.7, respondeo). Maimonides’ analysis of attributes as “superadded” simply cashes out, in the special case of divine predication, what that Thomistic axiom says universally about composites: where there is a many that is truly one, a unifying factor is in play; and if that unity is caused, the subject is not first. For Maimonides the only way to block that outcome is to deny that divine “attributes” are really distinct positives at all. The threat of a plurality of uncaused eternals—as many as there are supposed attributes—restates Aquinas’ unifier argument in a theological idiom. (New Advent)

At the same time Maimonides’ via negativa is not a counsel of quietism. Within classical theism, Thomas Aquinas pairs an equally strong simplicity thesis with an account of how affirmative names may still be truly said of God without multiplying essences. The key is that such names are said analogically and signify in God what is identical with the divine essence, not an accident really distinct from it. Thus, when we say “God is good,” we do not ascribe to God an added determination alongside what God is, but indicate that what “goodness” positively names in creatures exists in God “in a more excellent way.” Aquinas writes: “these names are applied primarily to God rather than to creatures, because these perfections flow from God to creatures; but as regards the imposition of the names, they are primarily applied by us to creatures which we know first.” He adds that among the perfections, “the first is existence, from which comes this name, HE WHO IS.” (Aquinas 1947–48, ST I, q.13, a.5, respondeo). This preserves the Maimonidean prohibition on adding constituents to God while explaining how language can remain realist rather than purely negative. What is predicated is identical in reality with the simple essence; the plurality lies only in our concepts. (New Advent)

The convergence with Avicenna further tightens the net. If, as Avicenna insists, “The First has no quiddity other than His individual existence,” then there simply is nothing in God for an “attribute” to be added to in the way attributes are added to creaturely essences (Avicenna 2005, Ilāhiyyāt VIII.4, p. 274). Maimonides’ ban on positive essential attributes is thus not a separate revelation-based restraint, but the theological expression of the same metaphysical insight: the first principle does not stand under a common measure, is not composed by any principle, and so cannot house many really distinct positives without ceasing to be ultimate. To call God wise, good, or powerful without division is to deny defect while recognizing in God, as first cause, the eminent source of the perfections signified by those terms in creatures. That is why Maimonides can say that positive attributes “imply polytheism”—they would amount to positing many uncaused firsts—and why Aquinas immediately adds that such names are true of God only in a mode that preserves simplicity and non-genus. (Internet Sacred Text Archive)

What follows for a classical adequacy test is straightforward. If one proposes a model of deity that enumerates really distinct essential perfections in God as a cluster of positive properties, one faces Maimonides’ dilemma: either those “attributes” are genuinely superadded and so yield composition, posteriority, and a cause of unity, or they are not really distinct and collapse into identity in the subject. The first horn abandons ultimacy; the second is classical simplicity. Maimonides codifies the metaphysical discipline necessary to avoid the first horn, and Aquinas shows how, within that discipline, affirmative predication can still be retained without generating a plurality in God. Thus stated, the principle that “There cannot be any belief in the unity of God except by admitting that He is one simple substance, without any composition or plurality of elements” (Maimonides 1904, Guide I.51, p. 69) is not a theological boundary marker only; it is the terminus of the very same arguments that forbid composition, genus-membership, and uncaused unity in any proposed first principle. (Internet Sacred Text Archive)

Bibliography (Chicago author–date; print-style; no links)
Aquinas, Thomas. 1947–48. Summa Theologiae. Translated by the Fathers of the English Dominican Province. 3 vols. New York: Benziger Brothers. I, q.3, a.7; I, q.13, a.5.
Avicenna. 2005. The Metaphysics of the Healing (al-Ilāhiyyāt). Translated by Michael E. Marmura. Provo, UT: Brigham Young University Press. Book VIII, chapter 4.
Maimonides, Moses. 1904. The Guide for the Perplexed. Translated by M. Friedländer. London: Routledge & Kegan Paul. Part I, chapters 50, 51, and 58.

Synthesis: Interlocking the Six Principles

These six principles form a tightly woven network. Pure actuality (1) and simplicity (3) are two sides of the same coin: disallowing potency means disallowing parts or change, which is simplicity. Cause-of-unity (2) and the composite‐causal principle (3) together exclude any internal plurality: if any plurality existed, it would need an extrinsic cause (Aristotle) or render God posterior to something (Aquinas), which is impossible. Non-genus (4) is linked with pure act: if God were a species under a genus, that would be a kind of addition. Aquinas explicitly ties non-genus to essence‐=‐existence. Avicenna’s pure existence (5) embodies both pure act and simplicity: God is existence, so again nothing extra. It also delivers non-genus from the necessity’s side. Maimonides’ unity (6) essentially assumes all earlier conclusions: only a simple pure being can sustain monotheism without implying a plurality of existents.

In a dependency graph: Aristotle’s insight on motion → “unmoved mover = pure actual” (1) feeds into Aquinas’s simplicity from uncausedness (3). Aristotle’s unity‐cause (2) also feeds into (3). Aristotle’s “being not genus” feeds into Aquinas’s non-genus (4) and Avicenna’s pure existence (5). Avicenna (5) reinforces (3) and (4): pure existence means simplicity and no genus. Maimonides (6) depends on (3)–(5): if God lacked simplicity and pure existence, His unity could not hold. The six taken together yield an adequacy criterion for God: the ultimate being must be pure act, simple, indivisible, outside all genus, non-composite – precisely the picture of God as classical theism conceives Him.

Implications spill into attributes: once God is pure act and simple, He must be immutable (no potency to change), eternal (exists outside time as Changeless), and omniscient/omnibenevolent in appropriate analogical senses. His knowledge and will flow from His essence as act of mind, not as additions. All the standard divine perfections (Simplicity, Pure Actuality, Oneness, Immutability) emerge naturally from these six. Importantly, none of this relies on scriptural authority – it is defended by philosophical reasoning from Aristotle onward.

Objections and Replies

A number of contemporary and historical objections can be raised. One is the modal collapse worry: if God’s essence is identical to His existence (as Avicenna and Aquinas claim), does that make all truths about God necessary and collapse contingency? Classical theists reply that God’s essence-existence identity means only that God’s existence is not dependent, not that everything is the same. God freely wills even though His nature is necessary; contingency resides in created beings whose existence isn’t identical to their essence. Another concern is the identity of attributes: if God’s attributes (wisdom, love, power, etc.) all are God’s essence, do they become identical and indistinguishable, making God’s knowledge and love the same? The answer is that on classical simplicity, God’s attributes are identical to His essence and each other, but from our perspective we use analogical terms. What seems like plurality is just one (unknowable) reality described variously. A third objection is the genus/differentia question: if God has no genus, how is any positive statement about Him possible? Here the interlocking principles guide us to the via negativa: we know what God is not (nothing composite, mortal, limited, etc.) and must speak of Him analogically. Though we must admit much mystery, the framework ensures no contradiction.

Other objections (some raised by medieval thinkers) include accusing Aristotle of circularity, or denying eternity of the world, or claiming the first principle’s simplicity leads to a worldless divine intellect. These are answered within the framework: if the first principle were composite or in a genus, it wouldn’t be first; if one denies a creator, one simply rejects the chain of causation that yielded these principles. In short, any alternative that avoids these principles tends to undermine them: without pure actuality one can’t explain change; without cause-of-unity one can’t explain how things hold together; without simplicity one can’t avoid infinite regress of parts; without non-genus one can’t preserve ultimacy; without pure existence there’s no account of necessity; without unity one contradicts monotheism. Thus the classical system holds up to its critics by showing that denying any principle collapses the entire edifice.

Conclusion

Philosophically, the payoff is that classical theism emerges as a rigorously defined position: the first principle must be pure act, simple, non-genus, non-composite, necessary. Each principle we examined is indispensable: Aristotle’s and Aquinas’s arguments force pure actuality and unity, while Avicenna and Maimonides show that any more content would destroy God’s uniqueness. This network answers why classical theists insist on divine simplicity and uniqueness: any alternative violates basic metaphysical coherence. For ongoing debates (e.g. divine attributes, modal metaphysics), the six principles form an “adequacy criterion”: any proposed account of God must meet all six to be truly the God of classical theism. They ground God’s immutability, eternity, omniscience, etc., without appeal to scripture, answering skeptics that this concept of God is inescapable if one accepts Aristotle’s and the medieval metaphysicians’ starting points. In sum, the six principles together show that if there is an ultimate first cause, it must be exactly as classical theism says – utterly simple and one. This completes the philosophical construction, rooted in our close readings of Aristotle, Aquinas, Avicenna, and Maimonides, rather than dogmatic assertion.

Appendix A: Quote Bank

• [1] Aristotle 1933, Metaphysics XII.6, 1071b17–20: “There must, then, be such a principle, whose very essence is actuality.”
• [2] Aristotle 1933, Metaphysics XII.7, 1072a24–26: “…there is something which moves without being moved, being eternal, substance, and actuality.”
• [3] Aquinas 1947–48, Summa Theologiae I, q.3, a.7, respondeo: “every composite is posterior to its component parts, and is dependent on them… every composite has a cause.”
• [4] Aristotle 1933, Metaphysics VIII.6, 1045a8–12: “What is the cause of their unity?… the whole is something besides the parts, there is a cause.”
• [5] Aquinas 1947–48, Summa Theologiae I, q.3, a.5, respondeo: “…being cannot be a genus… It follows then that God is not in a genus.”
• [6] Avicenna 2005, Ilāhiyyāt VIII.4, p. 274: “The First has no quiddity other than His individual existence.”
• [7] Maimonides 1904, Guide of the Perplexed I.51, p. 69: “There cannot be any belief in the unity of God except by admitting that He is one simple substance, without any composition or plurality of elements…”

Appendix B: Formal Reconstructions

Pure Act (Principle 1)
P1: Every change or motion requires a cause that is fully actual. (From Aristotle’s Physics/Metaphysics.)
P2: The chain of movers cannot regress to infinity, so there is a first cause.
P3: The first cause is unmoved (Aristotle: moves “without being moved” [2]).
C: The first cause must have an essence of pure actuality (no potentiality) [1].
Cause of Unity (Principle 2)
P1: In any composite whole, the whole is something over and above its parts. (Aristotle Met. VIII.6.)
P2: Whenever the whole is more than the sum of parts, there must be a cause that makes them one (“the whole is something besides the parts, there is a cause” [4]).
C: Therefore every plurality requires a cause of unity beyond the mere parts. (Formal cause, etc.)
Simplicity from Non-Posteriority (Principle 3)
P1: Every composite thing is ontologically posterior to and dependent on its parts (Aquinas: “posterior to its component parts” [3]).
P2: Every composite thing has a cause of its composition (Aquinas: “every composite has a cause” [3]).
P3: The first principle is uncaused and prior by definition (God as first cause).
C: Hence the first principle cannot be a composite (it is not posterior to or caused by parts), so it is absolutely simple.
Not in a Genus (Principle 4)
P1: If God were contained in a genus, that genus would have to be being, since God’s essence is pure existence.
P2: Aristotle shows “being cannot be a genus” (no genus can have differences beyond itself) [5].
C: Therefore God is not in any genus (Aquinas: “God is not in a genus” [5]).
Necessary Existent’s Pure Existence (Principle 5)
P1: Possible beings have a quiddity (essence) distinct from their existence.
P2: The Necessary Existent’s existence is by definition self-subsisting.
P3: Avicenna: “The First has no quiddity other than His individual existence” [6].
C: Therefore God’s only quiddity is existence itself. He is pure existence, not composed of essence+existence and not belonging to any genus.
Unity of God (Principle 6)
P1: If God had multiple essential attributes (plural elements), there would be multiple eternal essences.
P2: Maimonides: true belief in divine unity requires God to be “one simple substance, without any composition or plurality” [7].
C: Therefore God must be one simple substance. Any plurality of essential attributes is ruled out by (3)–(5), so God’s unity demands absolute simplicity.
Appendix C: Terminology Cross-Reference
• energeia (Greek) – Actuality. Aristotle’s term (Latin actualitas) denoting full being or activity as opposed to dunamis (potency).
• dunamis (Greek) – Potentiality. The capacity for change, distinct from actuality.
• actus (Latin) – Act. Aquinas’s Latin usage corresponding to Aristotle’s energeia.
• potentia (Latin) – Potential. Aquinas’s term for what is possible but not actual.
• wājib al-wujūd (Arabic) – Necessary Existence/Being. Avicenna’s term for the first cause whose existence is essential.
• māhiyya (Arabic) – Quiddity/Essence (“whatness”). The defining essence of a thing. In Avicenna, possible beings have a māhiyya distinct from existence, but God has no māhiyya beyond existence.
• inniyya/anniyya (Arabic) – Individual Essence/Esse per se. The individuating nature of an existent. Avicenna says God’s innīyya is pure existence itself.
• ens/esse (Latin) – Being/Existence. Aquinas uses ens for being-as-essence and esse for being-as-act; he teaches that in God these coincide (esse ipsum subsistens).
• genus/differentia – The logical framework for classification: a genus is a broader category, a differentia is the distinguishing feature. Classical theism denies God is a species under any genus.
Bibliography (primary sources first)
Aristotle. 1930. Physics. Translated by R. P. Hardie and R. K. Gaye. In The Works of Aristotle, edited by W. D. Ross. Oxford: Clarendon Press. (Book VIII.)
Aristotle. 1933. Metaphysics. Translated by W. D. Ross. Oxford: Clarendon Press. (Books VIII, IX, XII.)
Aquinas, Thomas. 1947–48. Summa Theologiae. Translated by the Fathers of the English Dominican Province. 3 vols. New York: Benziger Brothers. (Part I, q.3, aa.5–7.)
Avicenna. 2005. The Metaphysics of the Healing (al-Ilāhiyyāt). Translated by Michael E. Marmura. Provo, UT: Brigham Young University Press. (Books VIII–IX.)
Maimonides, Moses. 1904. The Guide for the Perplexed. Translated by M. Friedländer. London: Routledge & Kegan Paul. (Part I, chaps. 50–51, 58.)
Owens, Joseph. 1978. The Doctrine of Being in the Aristotelian Metaphysics. 2 vols. Leiden: E. J. Brill.
Peter Adamson. 2002. Avicenna. London: Routledge.
Leftow, Brian. 2002. Creation from God. Oxford: Oxford University Press.
Wippel, John F. 2000. The Metaphysical Thought of Thomas Aquinas. Washington, DC: Catholic University of America Press.
Davies, Brian. 1992. The Thought of Thomas Aquinas. Oxford: Clarendon Press.
Feser, Edward. 2008. Five Proofs of the Existence of God. San Francisco: Ignatius Press.
Kretzmann, Norman, Eleonore Stump, and Michael Boylan, eds. 1997. Cambridge Companion to Aquinas. Cambridge: Cambridge University Press.

Endnotes (sources for bracketed numbers)

[1] Aristotle, Metaphysics XII.6, 1071b17–20 (Ross trans.), Internet Classics Archive, “Metaphysics, Book XII.”
[2] Aristotle, Metaphysics XII.7, 1072a24–26 (Ross trans.), Internet Classics Archive, “Metaphysics, Book XII.”
[3] Thomas Aquinas, Summa Theologiae I, q.3, a.7, respondeo (Fathers of the English Dominican Province trans.), New Advent or Isidore edition (online ed.).
[4] Aristotle, Metaphysics VIII.6, 1045a8–12; 1045a33–b7 (Ross trans.), Internet Classics Archive, “Metaphysics, Book VIII.”
[5] Thomas Aquinas, Summa Theologiae I, q.3, a.5 (sed contra; respondeo; ad 2), New Advent or Isidore edition (online ed.).
[6] Avicenna, The Metaphysics of the Healing (al-Ilāhiyyāt), trans. Michael E. Marmura (Provo, UT: BYU Press, 2005), VIII.4–5, pp. 273–279 (print edition consulted).
[7] Maimonides, The Guide for the Perplexed, trans. M. Friedländer (London: Routledge & Kegan Paul, 1904), I.51, p. 69; see also I.50 and I.58 (CCEL online ed.).