It is this that we ought to mean when we say that we reason from the impossibility of the contrary. The contrary is impossible only if it is self-contradictory when operating on the basis of its own assumptions. It is this too that we should mean when we say that we are arguing ad hominem. We do not really argue ad hominem unless we show that someone’s position involves self-contradiction, and there is no self-contradiction unless one’s reasoning is shown to be directly contradictory of or to lead