Whence arises this trust in the uniformity of the phenomena of the Universe? We can hardly give any other answer except that we could not get on without it. Having been found to “work” by ourselves, and by many generations of our forefathers, this faith is possibly by this time an inherited instinct as well as the inbred result of our own earliest experiences. But when we analyse it we are forced to confess that we can give no logical account of it. Logically regarded, it savours of the most audacious optimism, arguing, or rather sentimentalizing, after this fashion: “It would be so immensely inconvenient if Nature were every moment changing her rules without notice! All forethought, all civilization would be at an end; nay, we could not so much as take a single step or move a limb with confidence, if we could not depend upon Nature!” Does not this personification of Nature, and trust or faith in Nature, somewhat resemble our trust or faith in God? I think it does; and it is very interesting to note that the very foundations of science are laid in a quasi-religious sentiment of which no logical justification can be given. I might easily go further and shew that, even as regards the past, we act in our daily lives very often on the grounds of faith and very seldom on the grounds of demonstration. On this I have touched in a previous letter; but your dictum about the “immorality of believing what cannot be proved” makes it clear that you are hardly as yet aware of the nature of the ordinary “proofs” on which we act. How few there are who have any grounds but faith for believing in the existence of a Julius Cæsar or an Alexander! Yet they believe implicitly. Many have heard these two great men loosely spoken of, or alluded to; but they have never weighed, nor have they the least power to weigh, the evidence that proves that Cæsar and Alexander actually existed. Now as the unlearned are quite certain of the existence of a Julius Cæsar, so are you too quite certain of many facts upon very slight grounds. You ask one man his name; another, how many children he has; a third, the name of the street in which he lives, and so on; how certain you often feel, on the slight evidence of their answers (unless there be special grounds for suspecting them) that your information is correct! The reason is that all social intercourse depends on faith; if you began to suspect and disbelieve every man who gave you answers to such simple questions as these, social life would be at an end for you, and you might as well at once retire to a hermitage; scepticism in matters of this kind has not worked, and faith has worked; and this has gone on with you from childhood and with your forefathers from their childhood for many generations. Thus faith has become a second instinct with you, and you act upon it so often and so naturally that you are not aware of the degree to which it influences and permeates your actions. The cases in