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. CHAPTER II THE NATURE OF FAITH (Attempts to show what we can prove by our reason, and what we know intuitively; what is implied in the process of thinking, and without which no thought could be.) The primary fact that we know about life is growth. Herbert Spencer has defined this growth, or evolution, in a string of long words which may be summed up to mean: the process whereby a number of things which are simple and like one another become different parts of one thing which is complex. If we observe this process in ourselves, and the symptoms of it in others, we discover that when it is proceeding successfully, it is accompanied by a sensation of satisfaction which we call happiness or pleasure; also that when it is thwarted or repressed, it is accompanied by a different sensation which we call pain. Subtle metaphysicians, both inside the churches and out, have set themselves to the task of proving that there must be some other object of life than the continuance of these sensations of pleasure which accompany successful growth. They have proven to their own satisfaction that morality will collapse and human progress come to an end unless we can find some other motive, something more permanent and more stimulating, something "higher," as they phrase it. All I can say is that I gave reverent attention to the arguments of these moralists and theologians, and that for many years I believed their doctrines; but I believe them no longer.