I, q. 48, a), "First act is the form and integrity of the thing; but second act is operation." Therefore operation necessarily presupposes actuality, as that which is second follows that which is first. All change, then, implies in its subject a potentiality or capacity to receive or to lose a perfection, and in the agent the existence eminently, formally, or virtually, of the perfection which it is to confer upon the subject, or of which it is to deprive it. Hence no subject can be the total cause of its actuation by some perfection, though it may, as do the vital faculties, concur to effect a change. Yet even here the faculty is not the adequate but the partial cause of cognition. We cannot, however, rest in the consideration of secondary movers only, the last of which depends on one preceding, and this in turn upon one prior to both; for all these are intrinsically dependent upon an external influence for their motion. How far soever the series be protracted, its first member is always dependent, nor can all the members be otherwise classed than as first, last, and intermediate. Therefore there must be a first independent entity producing motion. Now, this first mover contains in itself the perfections that it realizes in the various subjects of change. Hence it contains the various perfections of which the human intellect and will are capable; it must, therefore, be intelligent and free. The first mover unmoved by extrinsic agency is personal and infinite; it is God. But God, as the prime source of motion, is unmoved in the Aristotelian sense, that His infinity excludes all potentiality; He is immutable; He is most pure act. Yet He is moved in the Platonic sense of being the perfection of life and intelligence.
St. Thomas' second proof is taken from the nature of efficient cause. He argues after this manner: "We observe that in the sensible world order reigns among efficient causes. Yet we neither see, nor indeed is it possible, that anything is its own efficient cause, for it would then pre-exist to itself. Now, of efficient causes there cannot be an infinite series, for in a progressive series of subordinated causes, the first is cause of the mediate, and this of the last, whether the mediate be one
