An analysis of necessary condition and presupposition reveals that, as logical relations, these notions are basically similar to each other and different from the notion of entailment or other ‘if-then’ relations of logical consequence. Both necessary condition and presupposition seem to be two-directional in a rather peculiar way. Appreciating this is helpful in interpreting philosophers such as Kant and Strawson who have relied extensively on these relations in constructing the philosophical arguments often referred to as transcendental arguments. It also suggests some fundamental shortcomings in Strawson's account of presupposition and in the logician's way of presenting necessary condition using ‘⊂'.

In the discussion which follows I shall exhibit some of the differences between necessary condition and presupposition on the one hand and entailment on the other. I shall then go on to offer an explanation of these differences in conjunction with an analysis of necessary condition and presupposition which diverges from other contemporaty accounts. Once these distinctions and explanations have been stated, I shall make some suggestions about their relevance to the interpretation and assessment of philosophical texts and arguments.