If you haven't read Gödel Escher Bach - Eternal Golden Braid, then stop. Go Read.
Its by Douglas Hofstadter.
You can find it on esnips.com
the answer to the river and man question:
No! Because it can never be the same man nor the same river. The two in question having changed since the first instant.
If you don't lke the answer, don't blame me. Its accepted knowledge.