 Let us imagine David Hume, alone in his study, on a particular afternoon in 1775, thinking to himself, “I wrote the Treatise”. Can anyone else apprehend the thought he apprehended by thinking this? First note that what he thinks is true. So no one could apprehend the same thought, unless they apprehended a true thought. Now suppose Heimson is a bit crazy and thinks himself to be David Hume. Alone in his study, he says to himself, “I wrote the Treatise”. However much his inner life may, at that moment, resemble Hume’s on that afternoon in 1775, the fact remains: Hume was right, Heimson is wrong. Heimson cannot think the very thought to himself that Hume thought to himself, by using the very same sentence. (Perry 1977: 17)