What is it that we know about life? We know a seemingly endless stream of sensations which manifest themselves in certain ways, and seem to inhere in what we call things and beings. We observe incessant change in all these phenomena, and we examine these changes and discover their ways. The ways seem to be invariable; so completely so that for practical purposes we assume them to be invariable, and base all our calculations and actions upon this assumption. Manifestly, we could not live otherwise, and the spread of scientific knowledge is the further tracing out of such "laws"--that is to say, the ways of behaving of existence--and the extending of our belief in their invariability to wider and wider fields.

Once upon a time we were told that "the wind bloweth where it listeth." But now we are quite certain that there are causes for the blowing of the wind, and when our researches have been carried far enough, we shall be able to account for and to predict every smallest breath of air. Once we were told that dreams came from a supernatural world; but now we are beginning to analyze dreams, and to explain what they come from and what they mean. Perhaps we still find human nature a bewildering and unaccountable thing; but some day we shall know enough of man's body and his mind, his past and his present, to be able to explain human nature and to produce it at will, precisely as today we produce certain reactions in our test-tubes, and do it so invariably that the most cautious financier will invest tens of millions of dollars in a process, and never once reflect that he is putting too much trust in the permanence of nature.

In many departments of thought great specialists are now working, experimenting and observing by the methods of science. If in the course of this book we speak of "certainty," we mean, of course, not the "absolute" certainty of any metaphysical dogma, but the practical certainty of everyday common sense; the certainty we feel that eating food will satisfy our hunger, and that tomorrow, as today, two and two will continue to make four.