 Berkeley’s criticism of Locke’s theory concerning substances is not carried out for its own sake. On the contrary, it is meant to establish what Berkeley thinks to be the unavoidable metaphysical consequences of a position that takes ideas “in the mind” to be the only material for the operations of the mind in its acquisition of knowledge. These metaphysical consequences consist in a thoroughgoing idealism or “immaterialism” with respect to the nature and constitution of things or substances. Berkeley’s way of establishing this result is open to many questions. However, the basic outline of his overall argument can be sketched thus: If existence is restricted to ideas (and minds) and if, what is undoubtedly the case, things or substances exist, then things or substances must be ideas (or minds) too. Now, as Locke has convincingly shown, we can have ideas of particular things or substances, e.g., gold and lead, humans and sheep, distinguished by our ideas of their various properties, but we have only a confused or obscure idea of substance in general, which we suppose to underlie whatever collection of ideas we take to be a thing or a substance of one kind or another. But if we cannot have any ideas of things or substances other than our ideas of their properties, which clearly exist in minds, then the only clear ideas of things that we have is as ideas, and in that case, if they do not seem to exist in our own individual, human minds, then things or substances must be ideas in some other non-human, i.e., divine mind. This divine mind cannot be itself an idea because it must be conceived as an active principle that can be the cause of ideas, a principle of which we can have no idea but only a “notion” (1710: Part I, §26, §27). Therefore, the very fact that we take things or substances to be real commits us to the claim that things are ideal entities perceived by the mind of God. Idealism, one could say, is the only tenable basis for a realistic stance for Berkeley, but it leads to a realism about minds, human and divine, rather than of what he always calls material substance. And if one is to accept his re-interpretation of causality as a purported relation between ideas in terms of his theory of marks and signs, in particular his theory that what we think of as ideas of objects are signs of (God’s plan for) future possible ideas for us (cf. 1710: Part I, §65 f.), then one also has to agree to idealism. In Berkeley’s view, the only alternative to idealism is not materialism but skepticism.