 The agnosticism with respect to the ultimate constitution of substances and things or of the fundamentum in re of “the ideas thereof” characteristic of Hobbes and Locke is challenged forcefully by George Berkeley (1685–1753), for whom their agnosticism becomes a form of skepticism and even impiety. In his Treatise Concerning the Principles of Human Knowledge (1710) he raises doubts about whether an agnostic stance along the lines of Hobbes and Locke can be upheld consistently if one thinks about the origin and the properties of ideas the way they do. Although in his Treatise Berkeley does not mention Hobbes at all and addresses Locke not by his name but by formulas like “esteemed philosopher” and “learned author” (1710: Introduction, §11) very few times, it is abundantly clear that he wants to confront especially Locke with a painful choice: either his conception of a substance or a thing has “no distinct meaning annexed to” it (1710: Part I, §17) and is nonsense, or he has to endorse not just epistemological agnosticism but full-blown idealism or, in Berkeley’s term, immaterialism. In other words, Berkeley’s point is that Locke cannot afford to be agnostic with respect to the metaphysical status of substances and things if he wants us to think of ideas as the immediate objects of human knowledge. Arthur Collier (1680–1732) would make a similar argument.