Note 280 — Chapter IX

We may look at it in this way. Let AC denote the truth of both A and C, ACʹ the truth of A and the falsity of C, and so on. Then there are four à priori possibilities, namely, AC, ACʹ, AʹC, AʹCʹ, one or other of which must hold good, but any pair of which are mutually inconsistent. The proposition If A then C merely excludes ACʹ, and still leaves AC, AʹC, AʹCʹ, as possible alternatives. In denying it, therefore, we must definitely affirm ACʹ, and exclude the three other alternatives. Hence the contradictory as above stated.