Imagine an external intelligence studying the methods by which earth-born creatures of various types adjust themselves to future circumstances. The most primitive method is, I suppose, no more than simple nervous reaction. The most developed method involves reasoned expectation. And between these two extremes our supposed observer would see a long series of intermediate forms melting into one another by insensible gradation. From the point of view of the argument I am endeavouring to present to you, this development is of the greatest interest. The creation of a capacity for expectation, and of an inclination to expect a future similar to the past, must be deemed one of the most remarkable triumphs of selection—if to selection it indeed be due. Here we have this irrational mimic of reason, starting from the simplest forms of response to external stimulus, improving them into such excellent imitations of inductive reasoning as those which lead a chick, no more than a few hours old, to reject food which it has once found nasty[11]; and finally evolving out of these humble beginnings a mode of inference which, according to empirical philosophy, is the true and only source of all our general knowledge, whether of nature or of man. It must be owned, indeed, that the attempt to treat instinctive expectation as a form of rational inference has been a lamentable failure. By no exercise of ingenuity can beliefs about what is not experienced be logically extracted from particular experiences, multiply them as you will. It is in vain that empirical philosophers attempt to give an air of rationality to this leap from the known to the unknown by the use of high-sounding logical titles. “Induction by simple enumeration” is doubtless an imposing name. But those who practise the thing are in no wise improving on their predecessor, the chick. Indeed they lag behind it. For the chick expects—but gives no reason; the empirical philosopher expects—and gives a bad one. II Expectation, then, if it is to be rational, can only be rationally extracted from experiences by the aid of one or more general principles. What principles are they? One of them, at all events, must be the regularity of nature. In some form or other, and to some degree or other, this is assumed in every scientific speculation and in every purposeful action reflectively performed. It is, as you may recollect, one of the “inevitable beliefs of common sense” to which I referred in my first lecture. But you may also recollect that in the same lecture I pointed out that inevitable beliefs, though we cannot avoid holding them in some shape, are, and have been, held in many shapes; shapes which vary with the changes in our general outlook on men and things. In what shape, then, should our belief in regularity now be held?