IT IS UNFORTUNATELY not customary to include sketches of the social background and consequences of philosophies in expositions of them. This is deplorable, because their social role is frequently an essential clue to understanding them. People do not think in a vacuum, and even if the content and direction of their thought is in part determined by rational considerations, by where the wind of argument and the force of reasons and evidence drive them, these factors never uniquely determine what people think. By this I mean not that people are incapable of overcoming their emotional, non-rational inclinations (this may or may not be true as well), but that it is in the very nature of thought that its course is not rigidly dictated by some inherent rules. Some evidence may be incontrovertible and inescapable, some inferences cannot be resisted, and in those cases "we can no other". But the choice of problems, the choice of criteria of solutions, of rigour, of permissible evidence, the selection of hunches to be followed up and of those to be ignored, the choice of the "language game" or of the "form of life", if you like--all these matters which make up a style of thought or the spirit of the times, are not dictated by an immovable reason, and they are at the very least influenced by the social and institutional milieu of the thinker.
Any sociologist of knowledge, wishing to trace the mechanism of the institutional and social influence on thought, could hardly do better than choose modern philosophy as his field of enquiry. It provides him with an area of thought where the social factors --the tacit choice of criteria of acceptability, for instance-- operate, if not in an experimentally ideal state of isolation, at least in greater purity than they generally do in other fields. Philosophy, quite patently and also self-confessedly, is not a kind of thought which stands or falls with factual evidence; nor is it a matter of operating (or ingeniously constructing