Deconstructing Aquinas’ Fifth Way: Sup-Optimal Nature

Deconstructing Aquinas’ Fifth Way: Sup-Optimal Nature January 15, 2019

Ficino (commenter here at ATP with a great interest in Thomas Aquinas) has kindly produced a critique of Aquinas’ Fifth Way, as famously exhibited in Summa Theologiae. This will be split into four parts. This is the third part, the first part (on the preliminaries) can be found here; the second, on formulating the argument and initial issues, can be found here; the third, on further fallacies, is here.

Let me set out Ficino’s previous formulation of Aquinas’ argument:

  1. There exists an x such that x lacks cognition and x operates for the sake of an end.
  2. For all x, if x lacks cognition, x is a natural body.
  3. For all x, if x is a natural body and x operates for the sake of an end, x operates by intentionality.
  4. For all x, if x operates by intentionality, x is directed by an intelligence. [I don’t think this premise has to retain “if x is a natural body,” since all cognizant things also are directed by intelligence]
  5. Therefore, for all x, if x is a natural thing [sc. non-cognizant], then x is directed by an intelligence.
  6. Therefore, there exists some y, such that, for all x, if x is a natural thing, x is directed by y, and y is intelligent.

Now we look to wrap things ups with a final few observations. Back over to Ficino:

c) I have already suggested that the ambiguity of “best” in “so as to attain that which is best” leads to a question-begging step. In fact, there are many cases where natural things in the broad sense, cf. 4.b.iv previously, fail to actualize their form. I shall give only one example: it seems that half or more of human embryos either fail to implant properly in the uterus or else spontaneously abort later on [Ed – see “God Loves Abortion!” here at ATP]. On Aristotelian-Thomistic terms, these potential humans have nutritive soul but do not actualize the functions of sensitive and/or rational soul that were conveyed as potential by the semen. It is pushing it to say that human reproduction attains its end even “for the most part.”

Even Aristotle recognized cases of weakness in nature, where a species’ structure makes operations necessary but not “best” or “good.” Because of the weakness of the males in certain species of insects, Aristotle says, “nature” is not able to use the males as free-standing instruments to generate new insects. Instead, the female has to insert a part into the male to get the heat and movement necessary for the female’s matter to generate, and nature does this “with difficulty” (GA I.22 730b24‒29). Males in some species lack testicles, not because it is best, but from necessity (GA I.6, 717b33‒35). Donkeys are of cold nature, and male donkeys that have not begun to generate offspring by the time they lose their first teeth never generate, “so close to being infertile is the body of donkeys” (GA II.8 748b7-11). Female birds that don’t brood their eggs are in a worse condition because they lack a quality that they should have by nature (GA III.2 753a15-17). When some birds like raptors lay a third egg, it almost always goes rotten because of the hot nature of the species (GA III.2 753a33). Aristotle gives many examples of weakness of vision in GA V.1, including night vision and weakness of eyes of certain colors, including blue eyes (V.1 779b12‒780a26).

I give these examples, not because I think Aristotle always got the science right, but to show that the Aristotelian tradition that Aquinas inherited provides counterexamples against Aquinas’ conclusion that the typical operations of all natural things are directed toward what is best. The above are typical for those species but, in Aristotle’s eyes, not best. Aristotle has the flexibility to admit cases where nature does not typically achieve that which is best, and he can even point to deficiencies in nature, as in the case of body parts of certain aquatic animals, PA III.8 671a16; IV.12 695b2; History of Animals I.1 487b24. The Thomist’s position, on the other hand, requires that all deficiencies in natural ordering be explained away. I recall reading long ago some physicist’s analogy of nature, not to a finely‒designed machine, but to a contraption kept from flying apart by baling wire.


I do not claim to have proved the conclusion of the Fifth Way false. Neither do I deny that its conclusion is an assertion about metaphysics. What I deny is that we are under logical obligation to believe it. Thomists tend to counter that if we reject Aristotelian-Thomistic metaphysics, reality is rendered unintelligible and scientific knowledge impossible. I see no reason to accept that consequence. We know heaps of things about reality—just not all. Meanwhile, the Thomist has not shown that brute fact cannot mark the boundary of our explanations. To me, reality is more intelligible with brute facts than it is under Thomism, where we have to explain away what look like a slew of quality control issues on the part of the divine, governing intellect.

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