In his Essays on the Intellectual Powers of Man, Thomas Reid offers a barrage of objections to the view, held by David Hume, that conceivability implies possibility. In this paper, I present Reid's first two objections to the ‘maxim of conceivability’ and defend Hume from them. The first objection concerns our ability to understand impossible claims, while the second concerns thoughts about impossible claims (such as, for instance, the thought that they are impossible). Reid's objections have special force against Hume because of his commitment to analyse all operations of the understanding in terms of conception. I argue that these objections fail, on the grounds that they presuppose substantive views about language which we are not warranted in attributing to Hume. Ultimately, Reid's objections help us draw out a set of intertwined issues about the language-mind relationship in Hume.