If there were no Y, there cannot possibly be Z. We go on,—
If Y, then certainly X;
If no X, then Y is impossible.
As we know Z in fact, we get back to Y; and, as we find Y, we retrogress to X.
And the retrogression continues, say till we reach B,—
If B, certainly A;
If no A, then B is impossible.
But, what are we to say of A?
If A then certainly—what?
If no what?—then A is impossible.