A is a because it is not b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, or z.
B is b because it is not a, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, or z.
C is c because it is not a, b, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, or z.
And so on until z.
A does not have an inherent meaning, only meaning because it differs to the other symbols. And by not having an inherent meaning it defers meaning. This is what Derrida had meant by differance. The choice of a different letter but identical pronunciation was to highlight differing, deferring and difference. The implication is that nothing has meaning present unto itself, only meaning via absence. He terms the mistake to think otherwise metaphysics to presence.
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