 There are at least two distinct ways in which a non-realist can reject the existence dimension of realism about a particular subject matter. The first of these rejects the existence dimension by rejecting the claim that the distinctive objects of that subject-matter exist, while the second admits that those objects exist but denies that they instantiate any of the properties distinctive of that subject-matter. Non-realism of the first kind can be illustrated via Hartry Field’s error-theoretic account of arithmetic, and non-realism of the second kind via J.L. Mackie’s error-theoretic account of morals. This will show how realism about a subject-matter can be questioned on both epistemological and metaphysical grounds.