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] except by its act. Now, in every created intellectual substance there are two principles, namely, its substance (or essence) and its existence, for it has been shown that its essence is not its existence." The same principles with a different application are to be found in his work De potentia: "Existence," he says, "is twofold: essential or substantial,. . . and accidental. . . . That which is in potentiality to substantial being is termed materia ex qua; that which is in potentiality to accidental being is materia in qua. Likewise, strictly speaking, that which is in potentiality to substantial existence is called materia prima; but that which is in potentiality to accidental existence is the subject. And, as whatever is in potentiality may be called matter, so that from which it has its existence, whether substantial or accidental, may be called a form." From these and cognate passages throughout the works of St. Thomas, we conclude that composition of potentiality and actuality, whether these constituents be styled essence and existence as in spiritual substances, or matter and form as in bodies, is a source of contingency, although, as the illustrious Doctor warns us (Contra gentiles, II, 54), "composition of matter and form is not the same as that of essence and existence." Now, "what is contingent must be determined to action by an extrinsic agency" (Sum. th., I, q. 19, a. c. ad. 5). But so extensive is St. Thomas' range of vision that he never loses sight of the first and ultimate principles of things. Even the contingent is stamped with the seal of its divine origin. "Contingent things," he explains, "may be viewed under two aspects: first, as contingent, and secondly, as containing a necessary element. For nothing is so absolutely contingent as not to include something of necessity. Thus when we say, Socrates runs, the proposition is indeed contingent; but the relation of running to motion is necessary. Now, the contingency of a thing arises from its matter, for the contingent is that which is capable of either existing or not existing. But potentiality pertains to matter, while necessity is included in the notion of form; for what is consequent on the presence of the form is