The conception of a “datum” cannot be made absolute. Theoretically, it should mean something that we know without inference. But before this has any definite meaning, we must define both “knowledge” and “inference”. Both these terms have been considered in earlier chapters. For our present purpose it will simplify matters to take account only of such knowledge as is expressed in words. We considered in Chapter XXIV the conditions required in order that a form of words may be “true”; for present purposes, therefore, we may say that “knowledge” means “the assertion of a true form of words”. This definition is not quite adequate, since a man may be right by chance; but we may ignore this complication. We may then define a “datum” as follows: A “datum” is a form of words which a man utters as the result of a stimulus, with no intermediary of any learned reaction beyond what is involved in knowing how to speak. We must, however, permit such learned reactions as consist in adjustments of the sense-organs or in mere increase of sensitivity. These merely improve the receptivity to data, and do not involve anything that can be called inference. If the above definition is accepted, all our data for knowledge of the external world must be of the nature of percepts. The belief in external objects is a learned reaction acquired in the first months of life, and it is the duty of the philosopher to treat it as an inference whose validity must be tested. A very little consideration shows that, logically, the inference cannot be demonstrative, but must be at best probable. It is not logically _impossible_ that my life may be one long dream, in which I merely imagine all the objects that I believe to be external to me. If we are to reject this view, we must do so on the basis of an inductive or analogical argument, which cannot give complete certainty. We perceive other people behaving in a manner analogous to that in which we behave, and we assume that they have had similar stimuli. We may hear a whole crowd say “oh” at the moment when we see a rocket burst, and it is natural to suppose that the crowd saw it too. Nor are such arguments confined to living organisms. We can talk to a dictaphone and have it afterwards repeat what we said; this is most easily explained by the hypothesis that at the surface of the dictaphone events happened, while I was speaking, which were closely analogous to those that were happening just outside my ears. It remains _possible_ that there is no dictaphone and I have no ears and there is no crowd watching the rocket; my percepts _may_ be all that is happening in such cases. But, if so, it is difficult to arrive at any causal laws, and arguments from analogy are more misleading than we are inclined to think them. As a matter of fact, the whole structure of science, as well as the world of common sense, demands the use of induction and analogy if it is to be believed. These forms