Epistemic possibility and the Liar

Here’s a fun Liar paradox involving epistemic possibility. Say that a proposition p is epistemically possible if it is consistent with all you know.

Construct a sentence G such that:

0. G is true if and only if G is not epistemically possible.

E.g., “The proposition expressed by the first sentence in this post found in quotation marks is not epistemically possible.”

Now, you only know truths, and truth is consistent with truth. Thus:

1. If G is true, then it is consistent with everything you know.

But G is true if and only if it is not epistemically possible. So:

2. If G is true, then it is not consistent with everything you know.

Hence:

3. G is not true.

But now that you’ve seen this argument, you surely are in a position to know G not to be true. Suppose you exploit this and indeed come to know G not to be true. But then we have a contradiction. For if you know G not to be true, then G is not epistemically possible, and hence by (0), it must be that G is true.