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 existence, except the succession of changing sensations and ideas of sensations which alone are presented to us. I can never directly encounter an external object. What I suppose to be outward is all in my mind. With a world thus imprisoned within me, is there any legitimate way through which I can assure myself of realities outside my mind, including other living persons who have their own sensations and ideas as much as I have mine, but numerically different from my private stock? If not, grave consequences seemed to follow: faith and hope must die out of human life. So Reid argued.

The Treatise of Human Nature was the scarecrow that warned Reid of the destructive consequences of this assumption, that we are shut up among our own ideas; as he had himself in youth been taught to believe, and by which he had been led on to Berkeley's conception of a wholly ideal material world. He now began to see this ideal material world of Berkeley in a new light. It seemed to leave him alone in the universe of his own mind. The ideal matter of Berkeley cannot, he argued, inform me of the existence of other living persons: what I have supposed to be other persons must be, like the rest, only some of my own ideas. If I allow that my own sensations and ideas are my only possible original data, I cannot from such transitory phantoms infer the real existence of other persons. With this starting-point as my only one, the whole universe naturally supposed to surround me—bodies and spirits, friends and relations, all dead things and all living persons—acknowledged by common judgment to have a permanent existence, whether I am having ideas of them or not—vanish at once. My private sensations and ideas can never really signify to me the existence of other conscious beings; only what