We find ourselves continually in contact and collision with Power that is external to ourselves individually; and we seem, too, to be exerting powers of our own: on the Power latent in the universe, our happiness or misery, our whole destiny, depends. What does all this mean? What is meant by the judgment that all changes in the universe are ‘caused’; and what is the origin of this judgment, or of the conception of ‘cause’ that is involved in it? Are the powers which we recognise in us and around us to be referred only to inanimate things as their centres; or only to living and self-conscious persons; or to both? Priestley, as an advocate of the claim of matter to account sufficiently for self-conscious life in man, and Kames and Priestley by their universal necessity, confirmed Reid’s disposition to inquire further into the nature and origin of the conception of power, and the judgment that every change presupposes what we ambiguously call a cause. His cousin Gregory, too, inquired about causation, and gave the results to the world in an _Essay on the Difference between the Relation of Motive and Action, and that of Cause and Effect in Physics_, which appeared in 1792. That every change must be caused, Reid regarded as a postulate of which the contradictory was absolutely inconceivable: it was necessarily implied in the common rational sense of Man, and, while incapable of logical proof, was that without which nothing else could be proved. That the things presented to our senses are themselves powerless; that all power is spiritual, referable only to Will; and that this is the common sense implicate of our physical and moral experience of changes, was the proposition which Reid now set himself to justify. Our experience of our own voluntary exertion is the only experience which directs us to the quarter in which power, properly so called, resides. A voluntary agent is our only example of a cause, or of that to which change must be finally referred as its responsible source. We can find no productive power in any inanimate thing. Matter, instead of being the universal cause, is in itself powerless, and can account for nothing. The material universe is virtually an external system of interpretable signs, regulated by non-material power. Take the following extracts from various letters of Reid’s to Gregory, from 1785 onwards,[25] in the unpublished paper on Power:—