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In the Summa, Aquinas frequently invokes (for lack of a better term) certain "logical-metaphysical presuppositions." Here are two examples drawn from I-II, qq. 90-91:

  • "that which is the principle in any genus, is the rule and measure of that genus"
  • "in every genus, that which belongs to it chiefly is the principle of the others"

Sometimes he gives justification for these kinds of presuppositions, and other times he doesn't. But in both cases they seem rather ad hoc, as if constructed on the fly to fill in logical gaps. Where is Aquinas getting these? Is he simply drawing them from Aristotle, or is he working with a formal set of presuppositions that were well-established in the 13th century? Reading suggestions on this topic would be much appreciated.

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St. Thomas states in Summa Theologica I-II q. 94 a. 2 co. that all what you call "certain 'logical-metaphysical presuppositions'" derive from the principle of non-contradiction, which derives from the notion of being and non-being:

the first indemonstrable principle is that "the same thing cannot be affirmed and denied at the same time" [i.e., the principle of non-contradiction] which is based on the notion of "being" and "not-being": and on this principle all others are based, as is stated in Metaph. iv


primum principium indemonstrabile est quod non est simul affirmare et negare, quod fundatur supra rationem entis et non entis, et super hoc principio omnia alia fundantur, ut dicitur in IV Metaphys

(source: this answer)


Réginald Garrigou-Lagrange, O.P., lists the following metaphysical first principles in his Reality: A Synthesis of Thomistic Thought Chapter 56: Realism And First Principles:

metaphysical principles: The principle of contradiction or identity,1380 that of sufficient reason,1381 that of efficient causality,1382 and that of finality.1383 These principles, we say, are true, because it is evident that they are primary laws, not only of our mind but of all reality. They are not merely existential judgments, but express objective and universal impossibilities. Never and nowhere can a thing simultaneously exist and not exist, can a thing be without its raison d’être, can a non-necessary thing exist without cause, can a thing act without any purpose. Metaphysical principles admit no exception.
1380. Being is being, non-being is non-being, or, being is not non-being.
1381. Everything that exists has its raison d’être, intrinsic or extrinsic.
1382. Every contingent being depends on an efficient cause.
1383. Every agent, including natural agents not endowed with cognition, acts for an end.

(source: this answer)

See also: A Scholastic List of Philosophical Axioms

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  • 6.28, "What is supreme in a genus is cause of everything in the genus", from the link in your comment seems more relevant in this case. – Conifold Mar 12 at 0:03
  • @Conifold Yeah, I'm wondering if Doubt's question is really about genuses particularly, since both his examples mention genus. – Geremia Mar 12 at 3:21

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