VII.
NECESSITY
We have seen that if material things can alone be treated of by science, if things which can be seen and handled are alone amenable to the methods of science, then there can be no science of mind, and no scientific laws to regulate mental phenomena. In the same way, if the field of evolution is completely filled by the redistribution of matter and motion, then there is no room left in the theory of evolution in which to accommodate the history of ideas or of morals, there is no evolution of thought or morality, no continuity between higher and lower in the intellectual development of man and the brute.
We may admit that the methods of mental and moral science, of sociology and political economy, are not identical with those employed by physics or chemistry or astronomy. But we cannot admit that the facts which, if not the proper study of mankind, are at any rate of the greatest interest to man, are not subject to or part of the process of evolution, and cannot be reduced to scientific law and order. The methods of the philosophical sciences may not be the same as those of the exact sciences; but neither are the methods of chemistry those of astronomy—just as the instruments of the astronomer are not those of the chemist. The exactness which is attained by those sciences that can apply the methods of mathematics to their subject-matter cannot be rivalled by philology or psychology. But it is not to all the material sciences that the mathematical methods can be applied: meteorology deals with matter in motion, but not yet with exactitude. The intangible and invisible, but none the less real, facts of our mental and moral experience can be measured to some extent by the statistics and averages and curves employed by the sociologist, the demographer, and political economist: the intensity of a desire may be estimated roughly and relatively by the "effective demand" for its object, the will to live by the number of suicides.
Again, we may admit that the laws of the exact or material sciences do not extend to mental science, without thereby forfeiting the right to subject mental phenomena to scientific investigation and analysis. Chemistry does not cease to be a science because chemical affinity cannot be exhibited as a case of the gravitation formula. Need psychology renounce the claim to be a science because the laws of the association of ideas cannot be deduced, say, from the laws of motion?
Of course, if science has no other object than to describe with mathematical accuracy the exact way in which material things move, if no method is scientific which does not result in such a formula, and if no generalisation, however true, is scientific which does not formulate motion in space, then, indeed, it is unscientific to talk of the evolution of mind and thought, of man and of society.
On the other hand, the movements of material things in space are facts of which we are aware, phenomena of which we are aware through our senses; in a word, they are sense-phenomena. We are aware of them as existing simultaneously and in combination, or as succeeding one upon another; and no truth, even of the most mathematical and exact of the sciences, does, or can do, more than express with mathematical exactness the precise conditions under which these sense-phenomena co-exist or follow one another, or the precise conditions without which such co-existence or sequence cannot take place. A mathematical science dealing with material things states only and always that certain sense-phenomena occur invariably and uniformly under certain conditions. The exact sciences move within the limits of the Uniformity of Nature and the law of Universal Causation; and their subject-matter consists of sense-phenomena, i.e. of things which, as known to science, are objects of perception to some mind.
But sense-phenomena are not the only mental phenomena of which we are aware: there are ideas which we do not see or handle, or smell or taste, but of which we are nevertheless distinctly conscious. Thought has its movement, ideas have their co-existence and sequences, the association of ideas has its laws. There is a uniformity of human nature as well as of external nature; there are conditions under which certain actions are always performed, and without which they would never be done. Whether the body of propositions in which these conditions are formulated be accorded or denied the name of science, matters little. But it is difficult to see what are the so great differences between these phenomena and sense-phenomena that make the latter amenable and the former insusceptible to scientific treatment. Is it that ideas are invisible? So is weight, yet the gravitation formula is scientific. Is it that thought is impalpable? So is colour, so is sound—yet there are optics and acoustics.
Be this as it may, what makes things material susceptible to scientific treatment is a quality which is not peculiar to them, but which is shared by them in common with things immaterial: it is that they are objects of which the mind is immediately aware, phenomena present to some consciousness, and that they are phenomena which appear in consciousness as co-existent and successive in certain definite uniform modes which can be detected by thought and formulated in general propositions, or laws.