Why, your whole life is full of beliefs—as certain as any beliefs can be—which it is impossible to demonstrate! When you got up this morning did you not believe that your razor would shave and your looking-glass reflect; that your boiling water would scald if you spilt it, and your egg break if you dropped it; and a score or two of other similar perfectly certain beliefs—all entertained and acted on in less than an hour, but all incapable of demonstration? But you maintain perhaps that “these beliefs are not beliefs, but knowledge based on the uniformity of the laws of nature; you know that the laws of nature are uniform, and therefore you knew that your razor would shave.” But how, I ask, do you know that the laws of nature are uniform? “By the experience of mankind during many thousands of years.” But how do you know that what has been in the past will be in the future—will be in the next instant? “Well, if a law of nature were broken—say, for example, the law of gravitation—the whole Universe would fall to pieces.” In other words, you and I would feel extremely uncomfortable, if we existed long enough to feel anything; but what does that demonstrate? Absolutely nothing. It would no doubt be extremely inconvenient for both of us if any law of nature observed in the past did not continue to be observed in the future; but inconvenience proves nothing logically. It is no doubt extremely inconvenient not to be able to believe that your razor will shave; but what of that? Where is the demonstration? And remember your own _dictum_, “It is immoral to believe what cannot be demonstrated.” Perhaps you may try to writhe out of this application of your own principle by the use of grand terms; “The Laws of Nature have been proved to be true by experiment as well as by observation; they have been made the basis for abstruse calculations and inferences as to what will happen; then the philosopher has predicted ‘this will happen,’ and it has happened. Surely no one will deny that this is a proof!” A proof of what? Of the future invariableness of the sequences of Nature? I shall not only deny, but enjoy denying, that it is a proof; if you mean by proof such a demonstrative proof as you obtain in a syllogism, where the premises are assumed, or in mathematics, where you are reasoning about things that have no real existence but are merely convenient ideas of the imagination. Believe me, this distinction of terms is by no means superfluous. You and your young scientific friends are continually confusing “proof” with “demonstration;” and you have one use of the word “proof” for religion and another for science. When you speak of religion, you say “it is immoral to believe in it for it cannot be _proved_” (meaning “demonstrated”); when you speak of science, you say, “This can be _proved_” (not meaning “demonstrated,” but simply “made probable,” or “proved for practical purposes”).