KNOWING WHETHER AND TELLING WHETHER

Mr. Body lies foully murdered, and the suspects are Green, Mustard, Peacock, Plum, Scarlet, and White. We may take it as settled that one of them did it, and only one. The question is whether Green did it, or Mustard did it, or Peacock, or Plum, or Scarlet, or White. Holmes is on the scene.

If Green did it, then Holmes knows whether Green did it or … or White did it if and only if he knows that Green did it. Likewise if Mustard did it, then Holmes knows whether … if and only if he knows that Mustard did it. Likewise for the other cases. In short, Holmes knows whether … if and only if he knows the true one of the alternatives presented by the ‘whether’-clause, whichever one that is.

Similarly for telling. In at least one principal sense, Holmes tells Watson whether Green did it, or Mustard did it, or Peacock, or Plum, or Scarlet, or White, if and only if Holmes tells Watson the true one of the alternatives presented by the ‘whether’-clause. That is: if and only if either Green did it and Holmes tells Watson that Green did it, or … or White did it and Holmes tells Watson that White did it.

This is a veridical sense of telling whether, in which telling falsely whether does not count as telling whether at all, but only as purporting to tell whether.