Whats in a definition?

The first chapter of Scott McCloud's Understanding Comics is all about trying to create a definition for comics in general. Now definitions are an incredibly tricky thing. He takes Eisner's definition "sequential art" (pg.5) and then begins to expand on it (pg.7). His definition and in fact much of the readings we had on the subject left words almost entirely out of the matter. One thing that really caught my attention though was on page 12 and 13 he shows two things he considers to be comics that have no panels, one by Eisner himself, and then on page 20 he assserts that one panel strips aren't comics. It seems like an inherint contridiction. It seems as though knowing a comic when you see one is a lot easier then defining the whole genre.