 Thus while we can speak of the contradictory of a proposition, Geach observes, we cannot (pace McCall 1967) speak of the contrary, but only of a contrary, of a proposition. As Humberstone (1986, fn. 6) points out in response to Geach’s critique of McCall, however, the lack of uniqueness “does not prevent one from exploring the logical properties of an arbitrarily selected contrary for a given statement”. For our purposes, the crucial logical properties of contrariety are that (i) the contradictory of a proposition \(A\) is not a contrary of \(A\) and that (ii) contrariety unilaterally entails contradiction: