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 hold for a moment a man’s waking attention—that is what Kant never tells us and never himself knew.

Clearer upon this question of perception is the position of Berkeley; we may therefore take him as a fair representative of those critics who seek to invalidate the discovery of material objects.

Our ideas, said Berkeley, were in our minds; the material world was patched together out of our ideas; it therefore existed only in our minds. To the suggestion that the idea of the external world is of course in our minds, but that our minds have constructed it by treating sensations as effects of a permanent substance distributed in a permanent space, he would reply that this means nothing, because “substance,” “permanence,” and “space” are non-existent ideas, i.e., they are not images in sense. They might, however, be “notions” like that of “spirit,” which Berkeley ingenuously admitted into his system, to be, mysteriously enough, that which has ideas. Or they might be (what would do just as well for our purpose) that which he elsewhere called them, algebraic signs used to facilitate the operations of thought. This is, indeed, what they are, if we take the word algebraic in a loose enough sense. They are like algebraic signs in being, in respect of their object or signification, not concrete images but terms in a mental process, elements in a method of inference. Why, then, denounce them? They could be used