doubtless of importance, but not to science. If it is proved to be false, not a single fact of science is shaken. The mechanical theory may be true or may be false, but in either case it is a metaphysical theory. If science is descriptive--descriptive of the uniform succession and co-existence of facts--then science no more proves the mechanical theory to be true than it proves the volitional theory to be true. Both are theories as to _why_ facts should succeed one another in the order described by science; and science does not undertake to prove the truth of such theories, nor does she wait for them to be proved or disproved. Many men of science, however, are also philosophers, and hold, as they are fully entitled to hold, that the mechanical theory is the true interpretation of nature. Now, "mechanics is the science of motion; we can assign as its object: to describe completely and in the simplest manner the movements which occur in nature."[13] On the mechanical theory, therefore, "the object of all science is to reduce the phenomena of nature to forms of motion, and to describe these completely and in the simplest manner ... the only complete description is that afforded by a mathematical formula, in which the constants are supplied by observation. This permits us to calculate those features or phases of phenomena which are hidden from our observation in space or in time."[14] This, we need hardly add, is in agreement with Mr. Herbert Spencer's view of the theory of evolution as a description of the process of the redistribution of matter and motion. It seems, then, that according to this particular metaphysical theory, which maintains the mechanical explanation of nature to be the true one, the object of all science is to describe (with mathematical accuracy, where possible) the movements of things in space. But science is universal; evolution extends to the whole cosmic process. Therefore, the only things with which science has to do, or which are factors in the cosmic process, are things moving in space. As a metaphysical argument this theory seems to us unsatisfactory. It converts, simply and illegitimately, the proposition sanctioned by common sense, that material things are real, into the proposition opposed to common sense, that all real things are material. It assumes, apparently unconsciously and certainly without proof, that the only things capable of scientific description are movements in space, the only laws in the universe mechanical laws.