Principle C. A potency of any ‘X’ must be specified or properly disposed in order to receive any given act
Principle C, like both Principle A and Principle B, applies to both the existents in a possible truncated world as well as to cognitive beings capable of intentional acts. This principle entails that natural entities have dispositional properties. The dispositions, moreover, are ‘tailor-made’ for specific acts, as the following text illustrates: ‘Indeed the two are really similar in that a potency is nothing more than a certain relationship to an act. Without this likeness, there would be no necessary correspondence between this act and this potency’ (Commentary on the Soul, no. 366). Principle C entails that every potency must be adapted to receive a specific act. In other words, there is an ontological relationship between a potency and the specific act that perfects that potency, which entails that a potency or disposition is ‘tailor-made’ for a particular act. In Aquinas’s ontology, for example, a category mistake results if one were to suggest that the form of an oak tree could be embodied with the matter that was disposed to receive the form of a mouse. In effect, Aquinas argues that dispositional properties intrinsic to a potency are conditions necessary for a form or an act to be instantiated in that potency. It follows that, in Aquinas’s ontology, there is no such ontological existent as ‘pure matter’. The only matter that exists is ‘natured matter, which one finds with a substantial form in a primary substance. Accordingly, ‘materia prima, a concept that occurs frequently in Aquinas’s texts, is used as a mental construct. It does not exist separated or by itself—‘fleshed out, as it were—in the truncated world. Dispositional properties account for change in the external world. Like Aristotle, Aquinas adopts biology as a paradigm of explanation; hence, change and process are endemic to explaining the external world and our awareness of that world.39
In Aquinas’s ontology, change is a reception of a different form with the ‘substratum’ or ‘subject’ remaining the same. In the Commentary on the Physics, Thomas distinguishes between two types of change:
- (a) substantial change;
- (b) accidental or incidental change.
In the first case, in the primary substance, a new essence results from the change, while in the second case, some new incidental property accrues to a primary substance with the essence remaining the same. The substratum or subject in substantial change is the prime matter however that might be analysed; in incidental change, however, the substratum or subject is the primary substance of a natural kind. In addition, Aquinas, following Aristotle, enumerates three kinds of incidental change: qualitative, quantitative, act and potency is one of the most fundamental and far reaching principles in Thomistic philosophy, having for St. Thomas an even wider application than for Aristotle from whom it emanated. The two notions are complementary and are practically synonymous with “being determined” and “being determinable” ’: The Christian Philosophy of St. Thomas Aquinas, trans. E. Bullough (St Louis: Herder, 1939), p. 78.
39 The mathematical paradigm of explanation, adopted by Plato following the Pythagoreans, returned in early modern philosophy with Descartes and is characteristic of much early analytic philosophy; this metaphilosophy is fundamentally foreign to Aristotle and Aquinas.
and change of place. Changes central to his philosophy of mind belong to the category of ‘qualitative’ change. Often such cognitive changes are classified as ‘alterations’.
In Aquinas’s ontology, therefore, since a primary substance is an individual thing, in the case of substantial change, a new kind of primary substance comes about in the world; in the second case, the same primary substance remains constant during the change, but some new incidental property accrues to it. In each instance of change, however, some ‘X’ or other is serving as a ‘substratum’. Nonetheless, it must be remembered that the ‘X’ as substratum for a substantial change never exists as a concretum itself. A concretum for incidental change—i.e. the individual primary substance—is always a compound. Principle C entails that any change, which has been defined by Aquinas as the reception of a form into a disposition or ‘matter, can occur only if there is an adequate relationship between the ‘substratum’ and the ‘received act’. Principle C, furthermore, applies to instances of both esse intentionale and esse naturale. Both categories fit under the rubric of a ‘thing’ receiving a different form—one category in nature and another in knowing. In other words, a form can be exemplified in two radically different potencies, one in the external order of things and the other in a knowing power. This ability for two fundamentally different exemplifications of a form is the root foundation for the distinction between esse intentionale and esse naturale.
One additional point needs to be made. The metaphysical criterion for the existence of a set of dispositional properties serving as a substantial form, it might be argued, is a counterfactual proposition. This claim is not the assertion that the counterfactual proposition itself is the ontological ground for a substantial form. Aquinas is more of a realist than that. However, the counterfactual proposition is the means or methodological move for determining whether or not a set of dispositions, which constitute the ontological structure of a primary substance, is present in any given existent. Simply put, the content of a disposition is not exhausted by the linguistic entity. The counterfactual proposition is the linguistic means for determining an ontological ground. The mid-twentieth-century writings of Everett J. Nelson on the ontological categories of causality and substance are instructive in this discussion of Aquinas’s ontology. Nelson used a counterfactual proposition as the means towards establishing the ontological necessity for the category of substance and the category of causality. Nonetheless, one should note that, in a similar vein, it is logically possible that Aquinas’s system is compatible with using a counterfactual proposition as the means towards establishing the ontological necessity for a substantial form determining a natural kind. Using Aristotelian terminology, the counterfactual proposition forces one to consider the distinction between essential and accidental properties. Because a substantial form determines the set of dispositional properties essential to a primary substance of a natural kind, the use of counterfactual propositions is a fruitful analytic device for determining the category difference between properties of substantial forms and properties of accidental forms. Aristotle spells out this distinction between essential predication and accidental predication in the Categories, where he distinguishes between properties ‘said of’ a substance and properties ‘found in’ a substance (Categories, 1a20-24).
-  Peter T. Geach, Mental Acts (New York: Humanities Press, 1957), 4-7. On the status of a counterfactual proposition relative to dispositional properties, Geach’s interpretation is more in line with Aquinasthan is Ryle’s.
-  Simply put, Nelson distinguished between ‘nomic universal propositions’ and ‘accidental universalpropositions’. An accidental universal proposition refers to a collection of individuals classed togetherbecause of an accidental property; e.g. ‘All the chairs in the room are blue.’ A nomic universal proposition,on the other hand, refers to a collection of individuals classed together because of an essential property; e.g.‘All human beings are rational beings.’ A primary substance occurring in nature would be an instance of amember of a natural kind. Only a nomic universal proposition will uphold a counterfactual proposition.With an accidental universal proposition, e.g. ‘All chairs in this room are blue’ the following counterfactualproposition will not hold: ‘If X were a chair in this room, then it would be blue.’ There is nothing about‘being a chair in this room’ that entails ‘being blue’. It just happens that all of the chairs in a given room areblue. On the other hand, given a nomic universal property, e.g. ‘All humans are rational beings’, the following counterfactual proposition will hold: ‘If S were a human being, then S would be rational.’ See Everett J.Nelson, ‘The Metaphysical Presuppositions of Induction, Proceedings and Addresses of the AmericanPhilosophical Association 40 (1967), 19-33.