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 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Take the following extracts from various letters of Reid's to Gregory, from 1785 onwards, in the unpublished paper on Power:—

'Power to produce an effect supposes power not to produce it; otherwise it is not power but necessity, which is incompatible with power taken in a strict sense.… I am not able