tricity, only about 100 feet per second. They have also established the fact of a chemical or molecular change in the brain corresponding with changes in mental states; and with great probability, also, a quantitative relation between these corresponding changes, and therefore a relation between them of cause and effect. In the near future we may do more: we may localize all the faculties and powers of the mind in different parts of the brain, each in its several place, and thus lay the foundations of a scientific phrenology. In the far-distant future we may do even much more: we may possibly connect every different kind of mental state with a different and distinctive kind of molecular or chemical change in the brain; we may find, for example, that a right-handed rotation of atoms is associated with love, and a left-handed rotation with hate. We may do all this, and much more. We may push our knowledge in this direction as far as the boldest imagination can reach, and even then we are no nearer the solution of this mystery than before. Even then it would be impossible for us to understand how brain-changes can produce even the simplest psychological phenomena such as sensation, consciousness, will. By no Tort of the mind can we conceive how molecular motion can produce sensation or consciousness. The two sets of phenomena belong to different orders—orders so different, that it is simply impossible to construe the one in terms of the other.
It is not thus with other groups of phenomena in relation to one another. The phenomena of motion, heat, gravity, light, electricity, chemical affinity—yea, also of vitality—have been, or may be, construed in terms of each other, and all in terms of molecular motion. Whether our present theories on this subject be true or not, may admit of doubt; but a true theory is at least conceivable; all these may conceivably be reduced to the same order. But no amount of knowledge nor strength of imagination will in the least degree help us to understand the mysterious causal relation between the molecular changes in the brain and the corresponding effects in the mind, or between changes in the mind and corresponding changes in the brain.
I wish to put this as clearly and as strongly as I can. Suppose, then, an infinite human knowledge—infinite in degree, but human in kind; suppose, in other words, an absolutely perfect science, such as was conceived and admirably expressed by Laplace—a science which had completely subdued its whole domain and reduced it to the greatest simplicity, so that the whole cosmos and its phenomena is expressed by a single mathematical formula, which, worked out with positive signs, would give every phenomenon which would ever occur in the future, or with negative signs every event which had ever occurred in the past. Even to such an infinitely perfect science the causal relation of molecular motion on the one hand to sensation, consciousness, thought, and emotion on the other, or vice versa, would still be utterly unintelligible. Like the essential nature of matter, or
