The idea of definiteness implies the idea of equality: for if there could be one exact quantum, why couldn't there be two? Two equal quanta? Two identical photons, for instance? But two identical photons would have to have identical histories, i.e.: they would have to be the same. "Identity" presupposes sameness, and "something" (supposing there is such a thing as a "thing") can only be identical with itself - can only be equal to itself. Therefore, no two things can be identical; they can, at most, be similar. But similarity is, of course, a relative concept: all things are similar, in so far as they are "things", and no two things are equal, as long as they're not - the same.