Aquinas and other Scholastic philosophers frequently make use of the principle ‘agere sequitur esse’ (Latin for “action follows being” ), which means “action follows being.” The underlying concept is that what a thing does must necessarily reflect what it is in some way. Because their structures are different, the functions of the eyes and ears are distinct. Plants take in nutrients, grow, and reproduce, whereas stones do none of these things, owing to the fact that the former are living things and the latter are nonliving things. So on and so forth. According to Aristotelian philosophy, the thesis that ‘agere sequitur esse’ can be understood as an application of the basic idea that the PPC expresses with respect to efficient causes in the context of what Aristotelian philosophers call formal causes. An efficient cause is the event that results in the occurrence of something or the modification of something. In other words, whatever is in the thing that changes or comes into existence must have been in some way involved in the total set of factors that brought about this change or existence, according to the PPC yet again. In this sense, the effect can never be greater than the source. A formal cause is the nature of a thing, or the characteristic that distinguishes it as the kind of thing that it is. For example, the nature of a human being is that he or she is a rational animal. As rational animals, we are born with certain characteristics and behaviours that flow or follow from our nature. For example, we are born with the ability to communicate through language. It is essentially stated by the principle ‘agere sequitur esse’ that these characteristics and activities are limited to their natural state, just as an effect cannot be extended beyond its efficient cause. As a result, a stone cannot exhibit characteristics and activities such as nutrition, growth, and reproduction because these are associated with living organisms. Fundamentally, the principle ‘agere sequitur esse’ states that these characteristics and activities cannot be extended beyond their natural scope, any more than an effect can be extended beyond its efficient cause. As a result, a stone cannot exhibit attributes and activities such as nutrition, growth, and reproduction because these are outside the scope of a stone’s natural capabilities. Anything capable of performing these functions would not be considered a stone in the first place.
The principle ‘agere sequitur esse,’ like the PPC, is derived from the principle ‘agere sequitur esse.’ If an effect has the potential to extend beyond its total efficient cause, then the portion of the effect that extends beyond the total efficient cause would have no explanation and would be unintelligible. In a similar vein, if a thing’s activities could extend beyond its natural state—for example, if a stone could take in nutrients or communicate—then this activity would be devoid of explanation and unintelligible to the observer.
The PPC is implicit even in the argumentation of some naturalistic philosophers who are otherwise hostile to the metaphysical views defended by thinkers such as Aquinas, as I mentioned previously. A similar statement can be made about the principle that ‘agere sequitur esse’. This principle is perhaps best known for its application by Aquinas himself, who argues that the human soul can survive the death of the body after the body has died. The following is his justification: Intellectual activity, which is one of the activities of the human soul, is (according to Aquinas, on independent grounds) essentially immaterial, according to him. For a material thing to engage in an immaterial activity, on the other hand, would be a violation of the principle ‘agere sequitur esse’. As a result, the human soul must be a non-material entity. Furthermore, because immaterial things, in contrast to material things, have no natural tendency to decay, the soul does not cease to exist when the material body does.
Of course, a materialist would disagree with the claim that intellectual activity is immaterial, but that is beside the point because mental properties are immaterial but are nonetheless properties of a material thing—namely, the brain—and that is irrelevant. Essentially, the theory is an attempt to acknowledge the flaws in materialist theories of the mind without being forced to accept the dualist view that the mind is an immaterial thing. Searle’s criticism of the theory is that it is inherently unstable. Once it is established that a mental property is something “over and above” the brain, Searle argues that such a property cannot be a property of the brain but must be “a separate thing, object, or non-property type of entity” in order to be recognized as such. On the other hand, if a mental property is truly a property of the brain, then it cannot be something that exists “in addition to” or “in addition to and above” the brain. Other opponents of property dualism have complained that it is puzzling how a completely material object such as the brain could be capable of generating immaterial properties of its own.(John R . Searle, “ W h y I Am N o t a Property Dualist” , in Philosophy in a New Century: Selected Essays (Cambridge: Cambridge University Press, 2008), 160.).
Needless to say, this dispute extends far beyond the scope of this book’s subject matter, and I make no attempt to resolve it by drawing attention to it here. In this case, the point is simply to demonstrate that, as with the PPC, so too with the principle ‘agere sequitur esse,’ what appears at first glance to be a vestige of Scholastic tradition that contemporary secular philosophers would reject is in fact something to which many of those philosophers are at least implicitly committed. In any case, as with the PPC, PC, and PSR, the principle ‘agere sequitur esse’ is well-founded and capable of being applied to questions in natural theology, regardless of what many contemporary philosophers happen to think.