The thesis to be proved is that Evolutionism is incompatible with Materialism. St. Thomas agrees with the thesis for several reasons. First, the vital principle (soul) is not the body (material). Second, the spiritual soul of man is not material. Third, matter is only a component of natural bodies. Therefore, matter is not the only reality, as the Materialists claim. And so, matter is not the only cause of the evolutionary process as the Materialists claim, thereby making mere Materialism incompatible to explain or to cause evolution. So Evolutionism is incompatible with Materialism.

Does St. Thomas maintain that the vital principle, or soul, is not the material body? Yes, St. Thomas affirms the error of those who make the soul the same as the material body. St. Thomas teaches: "Others are more in error, who make the soul to be the body. Their opinions, although diverse and varied, are all refuted together as follows. Living beings, since they are natural things, are composites from material and form. They are composed of body and soul, which makes living things actual. Therefore, it is necessary that one of these be form and the other matter. The body is not able to be form, because the body is not ‘in’ another, as in matter or in the subject. Thus, the soul is the form. Therefore, (the soul) is not the body, because no body is form" (Aquinas Summa Theologiae 1. 75. 1).³⁵

Does St. Thomas maintain that the spiritual soul of man is not material? Yes, St. Thomas affirms the spirituality of the human soul against the Materialists. St. Thomas notes: "It is necessary that the intellective soul act on its own, having its own operation without the help of the corporeal object. And because everyone acts in so far as in act, it happens that the intellective soul would have an essence per se not dependent on the body" (Aquinas In De Anima 1).³⁶

Does St. Thomas hold that matter is only one component of natural bodies? Yes, St. Thomas holds that matter, together with form, is one of the two constitutive elements of material things, and therefore "enter into the constitution of any material reality" (Aquinas In Phys. 2. 2). Matter and form constitute the intrinsic causes of a thing, which alone do not sufficiently explain the causes of a thing, since they need two extrinsic causes, agent and goal, as St. Thomas notes: "It is necessary then that beyond matter and form, that some other principle acts and this is called the efficient, or moving, or agent, or principle of movement. And thus as Aristotle says in the Metaphysics, every agent operates in so far as it tends to something, it is necessary that there be a fourth principle, that to which the mover looks, and this is called the goal" (Aquinas De Principiis Naturae 3. 350-351).³⁷

St. Thomas also explains how confusion might arise about the nature of matter for the Materialists. St. Thomas explains that the matter is an incomplete substance. St. Thomas also holds that form is also an incomplete substance. St. Thomas holds that the composite of matter and form is a complete substance. The mistake of the Materialists is the "substantification of the material form," which means that the Materialists make matter a complete substance (ens quod), instead of an incomplete substance (ens quo).

Does St. Thomas teach that matter is an incomplete substance? Yes, St. Thomas does, and he also notes that, in common speech, matter (incomplete existence) is sometimes used for the subject (complete existence). St. Thomas says: "Matter, accordingly, differs from subject, because the subject is not what has existence by reason of something added to it, but it has complete existence of itself (per se); just as man does not have existence through whiteness. But matter has existence by reason of what is added to it, since of itself it has incomplete existence. So it is that absolutely speaking the form gives existence to matter; but the accident does not give existence to the subject, rather the subject gives existence to the accident, although sometimes one is used for the other, namely matter for the subject, and conversely" (Aquinas De Principiis Naturae 4).³⁸

Does St. Thomas teach that form is also an incomplete substance? Yes, St. Thomas does, by teaching: "... if that form would not be subsistent, its being (esse) would only be in this, that it might be united to that whose form it is" (Aquinas De Potentia Dei 3. 9. ad 6).³⁹ Hoenen, of the Gregorian University, elaborates on this text by noting that "act" does not happen, and later be united to "potency." When act and potency form the composite, this is the very "becoming" of act. Those who wrongly believe that act happens and then is united to potency, make the error of "substantification of material form."⁴⁰ This means that the form would wrongly be considered a complete substance (ens quod) instead of an incomplete substance (ens quo). St. Thomas says, "The error of many about forms happens because they judge forms (ens quo) as if they were judging about substances (ens quod)" (Aquinas De Virtitutibus in Communi 11).⁴¹ St. Thomas again judges that the material form is not first in time or nature, but "forms begin to actually exist when the composite is formed, not that these forms become (fiant) essentially (per se), but only incidentally (per accidens) (through the composit)" (Aquinas Summa Theologiae 1. 45. 8. ad 1).⁴²

Does St. Thomas make the joining of matter and form into a complete substance, or composite? Yes, St. Thomas does consider the composite the complete substance and the cause of action. St. Thomas teaches: "All these opinions (Platonists, Avicenna, and others) seem to come from one root; they were seeking the cause of forms, as if these forms were acting by themselves (ens quod). But just as Aristotle proved (Aristotle Metaphysics 7. 26-28), what properly happens belongs to the composite...Just as ‘becoming’ belongs to the composite, and strictly speaking not to the forms, in the same way and correlatively ‘to act’ or ‘to cause’ does not strictly speaking touch the form but rather the composite. Just as the form is only incidently (per accidens) said to be able to become, so also the cause causing the form is only incidental (per accidens)" (Aquinas Summa Theologiae 1. 65. 4).⁴³