Such is, in brief, the argument. Now, the answer: We may do all we have supposed 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 of the relation of brain-changes and mental changes than we are now. Even then it would be impossible for us to conceive _how_ brain-changes produce mental changes or _vice versa_. Physical changes in sense-organs, transmitted along nerve-fibers, determine changes in brain-substance. So much is intelligible. But now there appear--how it is impossible to imagine--consciousness, thought, emotion, etc.--phenomena of an entirely different order, belonging to an entirely different world. So different, that it is impossible to imagine the nature of the nexus between, or to construe the one in terms of the other. Brain-cells are agitated and thought appears: Aladdin’s lamp is rubbed, and the genie appears. There is just as much intelligible causal relation between the two sets of phenomena in the one case as in the other. Now, this mystery is not of the nature of those which disappear under the light of knowledge. On the contrary, science only brings it out in sharper relief, and emphasizes its absolute unsolvableness. Suppose an absolutely perfect knowledge, perfect in degree, but human in kind. Suppose an ideally perfect science--a science which has so completely subdued its domain, and reduced it to such perfect simplicity, that the whole cosmos may be expressed in a single mathematical formula--a formula which, worked out with plus signs, would give every phenomenon and event which shall ever occur in the future, and with minus signs every phenomenon and event which has ever occurred in the past. Surely, this is an ideally perfect science. Yet, even to such a science, the relation of brain-changes to mental states would be as great a mystery as now. It would even come out in stronger relief, because so many other apparent mysteries would disappear. Like the essential nature of matter or the ultimate cause of force, this relation lies evidently beyond the domain of science. It requires some other _kind_ of knowledge than human to understand it.