In philosophy there are in fact very few and perhaps no valid logical impossibility or reductio ad absurdum proofs. The reason for this is that to produce such a proof we need to be able to map the relevant parts of our discourse on to a formal calculus in such a way as to enable us to move from given formula ‘q’ to a consequence of the form ‘p.~p’ and thence as a further consequence to ‘~q’.
