It is plain, then, I think, that if we are to find a reason for the assertion that some particular perception of mine would probably not exist, unless someone else were having or had had a perception of a kind which I can name, we must find a reason for _some_ generalisation. And it is also plain, I think, that in many cases of this kind the generalisation must consist in an assertion that when one man has a certain kind of perception, some other man generally has had some other perception or belief. We assume, for instance, that when we hear or read certain words, somebody besides ourselves has thought the thoughts, which constitute the meaning of those words; and it is plain, I think, that we have no reason for this assumption except one which is also a reason for the assumption that when certain words are heard or read, somebody generally has had certain thoughts. And my enquiry, therefore, at least includes the enquiry: What reasons have we for such generalisations as these? for generalisations which assert a connection between the existence of a certain kind of perception in one man, and that of a certain kind of perception or belief in another man? And to this question, I think, but one answer can be given. If we have any reason for such generalisations at all, some reason must be given, in one way or another, by observation--by observation, understood in the wide sense in which it includes "experiment." No philosopher, I think, has ever failed to assume that observation does give a reason for _some_ generalisations--for some propositions which assert that when one kind of thing exists, another generally exists or has existed in a certain relation to it. Even those who, like Hume, imply that observation cannot give a _reason_ for anything, yet constantly appeal to observation in support of generalisations of their own. And even those who hold that observation can give no reason for any generalisation about the relation of one man's perceptions to another's, yet hold that it _can_ give a reason for generalisations about the relation of some to others among a man's own perceptions. It is, indeed, by no means agreed _how_ observation can give a reason for any generalisation. Nobody knows what reason we have, if we have any, for supposing that it can. But _that_ it can, everyone, I think, assumes. I think, therefore, most philosophers will agree, that if we can find any reason at all for generalisations of the kind in which I am interested, a reason for _some_ of them at all events must be found in observation. And what I propose to ask is: What reason can be found in observation for even a single proposition of the kind I have described? for a proposition which asserts that when one man has one kind of perception, another man generally has or has had another.