In this paper I am using the word ‘logic’ in an extended sense. If anyone finds my use objectionable, he will at least find that nothing hangs on it; and if the word as used in this context conveys no information to any person, he will not be the poorer, because as he reads on, he will soon see the class of questions with which the paper deals.

Some of the points I will make will be familiar enough, but I think I do also break some new ground; and anyway I think there is an advantage in bringing these considerations together so as to produce a relatively full discussion in a short space.