People involved in a fruitful critical discussion of a problem often rely, if only unconsciously, upon two things: the acceptance by all parties of the common aim of getting at the truth, or at least nearer to the truth, and a considerable amount of common background knowledge. This does not mean that either of these two things is an indispensible basis of every discussion, or that these two things are themselves "a priori" and cannot be critically discussed in their turn. It only means that criticism never starts from nothing, even though every one of its starting points may be challenged, one at a time, in the course of the critical debate.