is very plain reasoning in a circle—from which Heaven defend us all, and particularly our neighbor! The only evidence of this sort that is confirmatory must be a posteriori. If, from the consideration of things in general, a universal principle is brought to light, then the agreement of subsequently discovered or subsequently considered phenomena with this principle is strong evidence in favor of the probable truth of the principle itself. These considerations apply nowhere perhaps with greater force than in the investigation of gravitation. The problem is practically where Newton left it, unless indeed it has been rendered even more difficult by our inheritance of debarring ideas. The concluding chapter of the first book is wisely given to a psychological study of the current view of gravitation. The basic principle is not far to seek. It lies in the doctrine that gravitation is proportional to mass, and that mass is constant. With this conception firmly fixed in the mind, the interpretation of the phenomena of gravitation becomes a foregone conclusion. Yet the present authors show that this conception is far from being a necessary deduction from the facts of gravitation, is indeed probably a false deduction. Their experiments and reasoning lead them to believe, and the belief is supported by other observers, that either mass is not constant, or else that gravitation depends upon other factors than simple mass and distance. This alternative, however, as we shall see, is merely verbal.
Many of the psychological considerations in this first book are very obvious, yet they are none the less necessary, and we commend most highly the skillful vestibule which Mr. Singer and Mr. Berens have constructed to their new Temple of Truth.
The second book has to do with first principles. These the authors find to be four—persistence, resistance, reciprocity, and equalization. The four primary principles represented by these terms are at the basis of the whole work, and of their truth and adequacy the authors express themselves as having no misgivings whatever. They believe that the entire phenomena of the visible universe can be explained by referring them to these principles.
The principle of persistence grows out of a consideration of Newton's first law of motion: "Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon." This is, of course, only the principle of inertia, which is simply another, and it has always seemed to us an unnecessary, statement of the principle of causation. Nothing happens without a cause. It is perhaps well to emphasize this principle when fighting superstitions and other hobgoblins; but in an intelligent world it may be taken for granted as one of those primary conditions of thought which need neither statement nor discussion. The habit of speak-
