Principle B. A potency as such can only he affected by some ‘X’ or other that is in act

Principle B is co-relative with Principle A. Various texts discuss this interrelationship between act and potency:

Actuality is not per se contrary to potency; indeed the two are really similar, for potency is nothing but a certain relationship to act. And without this likeness, there would be no necessary correspondence between this act and this potency. Thus, potency in this sense is not actualized from contrary to contrary, but rather from like to like, in the sense that the potency resembles its act. (Commentary on the Soul, no. 366)

The following passages note this co-relative connection: ‘a potency is actualized by something already in act’ (#373), and ‘A thing in potency is one that can be in act’ (Commentary on the Physics, lec. 3, no. 2). This interrelationship between potency and act applies equally to knowing situations, as the following passage indicates: ‘whenever a potential knower becomes an actual knower, she must indeed be actualized by what is already in act’ (Commentary on the Soul, no. 371).^[1]

• [1] Many traditional scholastic commentators have argued for the importance of the act-potency distinction in understanding Aquinas’s ontology and epistemology. One such example is Copleston’s classic exposition, Aquinas, where one finds the following account: ‘The foregoing outlines of the distinction betweensubstance and accident, matter and form, essence and existence, all of which illustrate in their several waysthe general distinction which runs through all finite being, namely, the distinction between act and potentiality’ See: F. C. Copleston, Aquinas (London: Penguin, 1955), 104. Likewise, Kenny writes: ‘The key concepts in Aquinas’s metaphysics are those of actuality and potentiality. He derives the notions obviouslyfrom Aristotle and from Aristotle’s commentators, but he applies them in new areas and with new degreesof sophistication’: Medieval Philosophy: A New History of Western Philosophy, vol. 2 (Oxford: ClarendonPress, 2005), 195. Gilson once argued in much the same vein. ‘The principle of the real distinction between