The problem of the origin of life has given rise in the last centuries to a debate between the Mechanicists and the Vitalists. This problem was not confronted directly and explicitly by St. Thomas.³⁷ Nevertheless, there are a number of locations in the writings of St. Thomas that are helpful in understanding and solving the question of life and its evolution. First, St. Thomas endorses the Aristotelian definition of life, which differs from the Mechanicist position that man is only like a machine.³⁸ Secondly, St. Thomas endorses the four causes of Aristotle, avoiding the mistake of Mechanicism, which at most admits only the efficient cause.³⁹ Third, St. Thomas explains two types of operations, as opposed to the exclusive view of transient operations (like local motion) held by Mechanicism.⁴⁰ Fourth, St. Thomas notes the error of the Mechanicists who think that forms (like extension for Descartes) are the same as substance.⁴¹

Does St. Thomas endorse the Aristotelian definition of life, unlike the Mechanicists who maintain no essential difference between organic and inorganic bodies? Yes, St. Thomas defines any living being as "a substance to which pertains according to its nature to move itself, or to bring itself in any way to operation" (Aquinas Summa Theologiae 1. 18. 2).⁴²

Does St. Thomas endorse the four causes of Aristotle, instead of just the efficient cause of some Mechanicists? Yes, St. Thomas endorses the Aristotelian classification of causes.⁴³ St. Thomas also endorses a hierarchy among the causes, so that, "Among the causes there exists the following order: the material is perfected by the formal cause, the formal by the agent cause, and the efficient by the final cause" (Aquinas Scriptum in Liber Sententiarum 4. 3. 1. 1. sol. 1).⁴⁴ Further, St. Thomas maintains that, "It is necessary that the causes are four. In fact, because cause is what gives to what follows the being of something (cum causa sit ad quam sequitur esse alterius) in the examination of that being (the being of the effect) there is able to be two different points of view, one absolute, the other relative. By considering being absolutely, the cause of the being is the form, because it is the form which makes the thing actual. By considering being relative to the potency for existing which it had before acquiring actuality, we have to admit two other causes , given that potency can pass to act only in virtue of something that is already in act. One is treating of the material and the agent. The agent has the function of reducing the material of the potency to act. But the action of the agent tends to something determinate, in fact every agent tends to what is appropriate for it. Now, that to which the action of the agent tends is called the final cause. So then it is necessary that there be four causes: formal, material, efficient, and final" (Aquinas In Phys. 2. 10. 240).⁴⁵

Does St. Thomas distinguish between transient and immanent operations, unlike the Mechanicists who admit only the transient operations? Yes, St. Thomas clearly makes such a distinction between transient and immanent operations.⁴⁶ St. Thomas teaches, "There are two kinds of operations. Some are transient (transiens) from one subject to another as heating from fire to wood...The other kind of operation is non-transient (non transiens) from some extrinsic cause, but remains in the same subject that acts, for example to sense, to know, to will and the like... The first kind of operation is common to living and non-living, while the second kind (not transient) pertains exclusively to living things (secundum operationum genus et proprium viventis). Because these operations are perfective of the operating subject" (Aquinas De Potentia Dei 10. 1). St. Thomas also distinguishes transient from immanent (not transient), saying "Quarum haec est differentia, quia prima actio non est perfectio agentis quod movet, sed ipsius movi, secunda autem actio est perfectio agentis" (Aquinas Summa Theologiae 1. 18. 3 ad 1).⁴⁷

Does St. Thomas expose the error (substantification or "reification"of the material form)⁴⁸ of the Mechanicists who consider forms as if these forms (ens quo) were the same as substances (ens quod)? St. Thomas says: "Many persons fall into error about forms because they consider them as they consider substances; which from this appears to happen that forms just like substances are labeled abstractly, like white or power, or something of that kind; whence following some figure of speech so judge them as if they actually were substances. From this then follows the error both of those who hold pre-existence of forms as well as those who hold forms to be created. They think that forms would have ‘becoming’ just as substances; and therefore not finding from where forms are generated, they say forms are created or pre-exist in material; not attending to the fact that just as "being" does not pertain to form (principium quo) but to the subject (ens quod) through the form, so neither does "becoming," which terminates at being (esse), so becoming is not of the form but of the subject. So also form is called being (ens) not because it is, if we speak properly, but because something is from it (ea), so also the form is said ‘to become’ not because it is becoming, but because from it (ea) something ‘becomes,’ when for example the subject is reduced from potency to act" (Aquinas De Virtutibus in Communi 11).⁴⁹