Mr. Bach,

I think you're right to point out that "number" meant a different thing to the Greeks; but I think that should make us more, not less, confident that "2+2=4." If the Greeks had meant the same thing by number as modern mathematicians do, than they were wrong to be very confident that the square root of negative one was not a number. However, the square root of negative one does in fact fall short of being a simple, definite multitude -- what Euclid, at least, meant by number. So if they were in error, it was the practical error of drawing an unnecessary distinction, not a contradictory one.

Perhaps "100% certain" or "P=1" could mean that I believe something to be true with the same level of certainty as that by which I believe certainty and probability to be coherent terms. We can only evaluate judgments if we accept "judgment" as a valid kind of thought anyway.