Arguments, at least the best of them, should be based upon principles of logic, and therefore be beyond dispute. But unfortunately many philosophical arguments are based upon principles which, though claimed by some to be principles of logic, or at least to be true, are disputed or rejected by others. This difficult position arises, no doubt, because it is the philosopher more than anyone else who is entitled to delve into questions of the validity of first principles. In a philosophical undertaking of any magnitude it is therefore imperative for the author to state as clearly as possible the principles which seem to him certain, and which constitute the guiding lines of his method. Any criticism could then take only one of two forms: (1) it could allege that the argument under criticism does not conform to the principles laid down, though these principles are themselves accepted. (2) it could allege that the argument is mistaken because something is wrong with the principles, in which case the object of criticism is properly the principles and not the argument. But a valid criticism could not take the form of condemning a certain argument in the light of a principle which, while appearing to the critic to be true and undisputed, is in fact inconsistent with the principles which the author has laid down.