I am proposing then to ask: Which among the observations, which I make, and which (as we commonly suppose) are similar in kind to those which all or almost all men make, will give a reason for supposing that the existence of any of them is generally connected with the existence of certain kinds of perception or belief in other people? And in order to answer this question, it is obvious we must first consider two others. We must consider, in the first place: Of what nature must observations be, if they are to give a reason for any generalisation asserting that the existence of one kind of thing is generally connected with that of another? And we must consider in the second place: What kinds of things do we observe? Now to the first of these questions I am not going to attempt to give a complete answer. The question concerning the rules of Inductive Logic, which is the question at issue, is an immensely difficult and intricate question. And I am not going to attempt to say, what kind of observations are _sufficient_ to justify a generalisation. But it is comparatively easy to point out that a certain kind of observations are _necessary_ to justify a generalisation: and this is all that I propose to do. I wish to point out certain conditions which observations must satisfy, if they are to justify a generalisation; without in any way implying that all observations which do satisfy these conditions, _will_ justify a generalisation. The conditions, I shall mention, are ones which are certainly _not_ sufficient to justify a generalisation; but they are, I think, conditions, without which no generalisation can be justified. If a particular kind of observations do _not_ satisfy these conditions, we can say with certainty that those observations give us _no_ reason for believing in the existence of other people; though, with regard to observations which _do_ satisfy them, we shall only be able to say that they _may_ give a reason.