In my previous post Synthesis-versus-Analysis I dealt with the distinction between “true whole” and “false whole”. Now let’s see how that had relations with Aquinas and his refutation of biological macroevolution.
About the origin of man and the relations between his soul and body, Aquinas was clear:
Reply to objection 3: Some have claimed that the [first] man’s body was formed antecedently in time, and that later on God infused a soul into the already formed body. But it is contrary to the nature of the perfection of the first production of beings that God would make either the body without the soul or the soul without the body; for each of them is a part of human nature. It is especially inappropriate to make the body without the soul, since the body depends on the soul, but not vice versa. [Summa Theologiae, 91, IV]
Note that the above quote especially applies to the negation of the arise of man from a non-human being (anthropoid). But in general denies the material macroevolution of any living being, because no being is inanimate (also if obviously human soul is incomparably higher than any animal soul) and Aquinas states that “soul is the form of the body” (in Scholasticism, in general, the “form” is the qualitative “principle” or “essence” of a thing):
Reply to objection 3: […] But since the soul is the form of the body, it does not have esse separately from the body’s esse; instead, it is united to the body directly through its own esse. [ibidem, 76, VII]
We can conclude that Aquinas is contra universal macroevolution in principle, because macroevolution is transformation of bodies only, while in Aquinas soul and body are not separable and the latter causatively depends on the former. By the way, this crystalline Aquinas’ position, shows how inconsistent are some Catholics (or even neo-Thomists!) who think to can believe, in the same time, in the Catholic doctrine (of which Aquinas is the master reference) and biological transformism.
But here I want to elaborate a bit specifically the above Aquinas statement: “But it is contrary to the nature of the perfection of the first production of beings that God would make either the body without the soul or the soul without the body”.
Beings are “perfect” because they are “true wholes”. If they are “true wholes” then their constitution / organization spiritus-anima-corpus must be an integrated “unit” or “oneness”. As I said in the linked post a “true whole” is a synthesis that can be neither produced nor conceived by analysis, rather only by means of “synthetic knowledge” (related to intelligent design). Because of such “synthetic knowledge” any kind of being is a top-down manifestation / instantiation of a metaphysical archetype into matter, by means of a vertical causation across the three layers: spiritual, animic (soul), corporeal (body).
Differently, a material macroevolution, or macro-morphing, of a being A to a being B would be a step-by-step analytic process, which — as we have seen — can never reach the limit of the target “true whole”. If the limit unit is not reached, and the beings are units, they neither can be produced by such analytic manner nor we can speak of “perfection”, neither about the process nor about its result. Goes without saying that such analytic process fails also because doesn’t work at all on the spiritual and animic planes.
As a consequence, only the above synthetic “vertical causation” can account for the “perfection of the production of perfect beings”, as Aquinas puts it. Any analytic serial horizontal macroevolution wouldn’t be “perfect” and wouldn’t produce “perfect” beings at all. This is the reason why Aquinas speaks of “perfection of the first production of beings” and coherently denies transformism.
Of course Aquinas’ cosmologic teachings about creatures’ origin, which are rigorously based on ontological principles, agree perfectly with the modern perspective of engineering. To provide a practical example, engineers never physically transform — say — cars into airplanes, rather they design in abstracto and assembly cars and airplanes independently. Also engineers apply an intelligent “vertical causation”, from abstract archetypes to material systems. No wonder, it couldn’t be otherwise because truth, at any level, is necessarily coherent, and the principles of intelligent design are universal.