Aristotle tells us: to say of what is that it is, or of what is not that it is not, is to state truth; to say of what is that it is not, or of what is not that it is, is to state falsehood. This seems to identify a substantive condition on stating either truth or falsehood: that it is either what is, or is not; which one presents as what is, or is not. There are reasons for thinking there is such a substantive condition. But state it, and, by standard application of logical principles, one can easily arrive in contradiction. This chapter suggests that that is because the presumption of satisfaction of this condition is already built into those standard applications. The standard principles are, in a sense, designed for such applications. When the presumption is made explicit, the idea of a substantive condition can stand. This bears on, though by itself does not decide, issues raised by Williamson about vagueness and about knowledge.
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