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 with all confidence to lead us back to the concrete values for which they stood and to the relations which they enabled us to state and discover. Experience would thus be furnished with an intelligible structure and articulation, and a psychological analysis would be made of knowledge into its sensuous material and its ideal objects. What, then, was Berkeley’s objection to these algebraic methods of inference and to the notions of space, matter, independent existence, and efficient causality which these methods involve? [Sidenote: Horror of physics.]