Jascha, you may also want to take a look here:
Relevance_Archives: RT list: Tautologies: 'A toothbrush is a to
Here's a peek:
Tautologies are interesting because they seem to hold no truth-content and so add no new information to a conversation, yet are widely used do carry a strong and obvious meaning. In particular, there are many common tautologies of the form 'X is X'. This example shows one such utterance in context:
A: 'I don't know what kind of toothbrush to get, there are toothbrushes
with flexible heads, toothbrushes with different shaped bristles,
toothbrushes hard bristles, soft bristles...'
B: 'A toothbrush is a toothbrush.'