...Descartes said haughtily, "Beer? I think not!" and -pouf!- he disappeared!

Logical fallacy:

The opposite of "A implies B" is not "not-A implies not-B" (isn't that the contrapositive?) but "B and not-A", which here is equivalent to "I am, but I'm not thinking!"

But fallacy, shmallacy. It makes a much better joke this way.

Do I make my point?


And while we're on the subject of what is and what isn't consider this:

AIN'T IT ISN'T

It’s is not, it isn’t ain’t, and it’s it’s, not its, if you mean it is. If you don’t, it’s its. Then too, it’s hers. It isn’t her’s. It isn’t our’s either. It’s ours, and likewise yours and theirs.
-- Oxford University Press, Express News (sorry I don't have a date for that)