2. But, it will be said, the necessary character claimed for such truths is an illusion. The propositions so brought into view are really established by observation: by the study of external facts: and it is only the effect of habit and familiarity which makes men of science, when they well know them to be true, think them to be necessarily true. They are really the results of experience, as their history shows; and therefore cannot be necessary and à priori truths.
To which I reply: Such principles as I have mentioned,—that material substance cannot be produced or destroyed—that the cause is measured by the effect—that reaction is equal and opposite to action: are not the results of experience, nor can be. No experience can prove them; they are necessarily assumed as the interpretation of experience. They were not proved in the course of scientific investigations, but brought to light as such investigations showed their necessity. They are not the results, but the conditions of experimental sciences. If the Axiom of Substance were not true, and were not assumed, we could not have such a science as Chemistry, that is, we could have no knowledge at all respecting the changes of form of substances. If the Axioms of Mechanics were not true and were not assumed, we could have no science of Mechanics, that is, no knowledge of the laws of force acting on matter. It is not any special results of the science in such cases; but the existence, the possibility, of any science, which establishes the necessity of these axioms. They are not the consequences of knowledge, acquired from without, but the internal condition of our being able to know. And when we are to know concerning any new subject contained in the universe, it is not inconceivable nor strange that there should be new conditions of our knowledge.
It is not inconceivable or strange, therefore, that as new sciences are formed, new axioms, the foundations of such sciences, should come into view. As the light of clear and definite knowledge is kindled in successive chambers of the universe, it may disclose, not only the aspect of those new apartments, but also the form and structure of the lamp which man is thus allowed to carry from point to point, and to transmit from hand to hand. And though the space illumined to man's vision may always be small in comparison with the immeasurable abyss of darkness by which it is surrounded, and though the light may be dim and feeble, as well as partial; this need not make us doubt that, so far as we can by the aid of this lamp, we see truly: so far as we discern the necessary laws of the universe, the laws are true, and their truth is rooted in that in which the being of the universe is rooted.
And, to dwell for a moment longer on this image, we may also conceive that all that this lamp—the intellect of man cultivated by science,—does, by the light which it gives, is this—that it dispels a darkness which is dark for man alone, and discloses to him some things in some measure as all things lie in clear and perfect light before the eye of God. To the Divine Mind all the laws of the universe are plain and clear in all their multiplicity, extent and depth. The human mind is capable of seeing some of these laws, though only a few; to some extent, though but a little way; to some depth, though never to the bottom. But the Human Mind, can, in the course of ages and generations, by the long exercise of thought, successfully employed in augmenting knowledge, improve its powers of vision; and may thus come to see more laws than at first, to trace their extent more largely, to understand them more thoroughly; and thus the inward intellectual light of man may become broader and broader from age to age, though ever narrow when compared with completeness.
3. Is it strange to any one that inward light, as well as outward knowledge, should thus increase in the course of man's earthly career? that as knowledge extends, the foundations of knowledge should expand? that as man goes on discovering new truths, he should also discover something concerning the conditions of truth? Is it wonderful that as science is progressive the philosophy of science also should be progressive? that as we know more of everything else, we should also come to know more of our powers of knowing?
This does not seem to have been supposed by philosophers in general; or rather, they have assumed that they could come to know more about the powers of knowing by thinking about them, even without taking into account the light thrown upon the nature of knowledge by the progress of knowledge. From Plato downwards, through Aristotle, through the Schoolmen, to Descartes, to Locke, to Kant, Schelling and Hegel, philosophers have been perpetually endeavouring to explore the nature, the foundations, the consequences of our knowledge. But since Plato, scarcely one of them has ever proceeded as if new light were thrown upon knowledge by new knowledge. They have, many or all of them, attempted to establish fundamental truths, some of them new fundamental truths, about the human mind and the nature and conditions of its knowledge. These attempts show that they do not deny or doubt that there may be such new fundamental truths. Such new fundamental truths respecting the human mind and respecting knowledge must be, in many cases at least, (as it will be seen that they are, on examining the systems proposed by the philosophers just mentioned,) seen by their own light to be true. They are new axioms in philosophy. These philosophers therefore, or their disciples, cannot consistently blame us for holding the possibility of new axioms being introduced into philosophy from age to age, as there arise philosophers more and more clear-sighted.
4. But though they have no ground for rejecting our new axioms merely because they are new, we may have good ground for doubting the value of their new axioms, that is, of the foundations of their systems; because they are new truths about knowledge gathered by merely exploring the old fields of knowledge. We found our hopes of obtaining a larger view of the constitution of the human mind than the early philosophers had, on this:—that we obtain our view by studying the operation of the human mind since their time; its progress in acquiring a large stock of uncontested truths and in obtaining a wide and real knowledge of the universe. Here are new materials which the ancients had not; and which may therefore justify the hope that we may build our philosophy higher than the ancients did. But modern philosophers who use only the same materials as the ancient philosophers used, have not the same grounds for hope which we have. If they borrow all their examples and illustrations of man's knowledge of the universe, from the condition of the universe as existing in Space and Time, that is, from the geometrical condition of the universe, they may fail to obtain the light which might be obtained if they considered that the universe is also subject to conditions of Substance, of Cause and Effect, of Force and Matter: is filled with Kinds of things, in whose structure we assume Design and Ends; and so on; and if they reflected that these conditions or Ideas are not mere vague notions, but the bases of sciences which all thoughtful persons allow to be certain and real.
It is then, as I have said, from taking advantage of the progressive character which physical science, in the history of man, has been found to possess, that I hope to learn more of the nature and prospects of the human mind and soul, than those can learn who still take their stand on the old limited ground of man's knowledge. The knowledge of Geometry by the Greeks was the starting-point of their sound philosophy. It showed that something might be certainly known, and it showed, in some degree, how it was known. It thus refuted the skepticism which was destroying philosophy, and offered specimens of solid truth for the philosopher to analyse. But the Greeks tried to go beyond geometry in their knowledge of the universe. They tried to construct a science of Astronomy—of Harmonics—of Optics—of Mechanics. In the two former subjects, they succeeded to a very considerable extent. The question then arose, What was the philosophical import of these new sciences? What light did they throw on the nature of the universe, on the nature of knowledge, on the nature of the human mind? These questions Plato attempted to answer. He said that the lesson of these new sciences is this:—that the universe is framed upon the Divine Ideas; that man can to a certain extent obtain sight of these Ideas; and that when he does this, he knows concerning the universe. And again, he also put the matter otherwise: there is an Intelligible World, of which the Visible and Sensible world is only a dim image. Science consists in understanding the Intelligible World, which man is to a certain extent able to do, by the nature of his understanding. This was Plato's philosophy, founded upon the progress which human knowledge had made up to his time. Since his time, knowledge, that is science, has made a large additional progress. What is the philosophical lesson to be derived from this progress, and from the new provinces thus added to human knowledge? This is a question which I have tried to answer. I am not aware that any one since Plato has taken this line of speculation;—I mean, has tried to spell out the lesson of philosophy which is taught us, not by one specimen, or a few only, of the knowledge respecting the universe which man has acquired; but by including in his survey all the provinces of human knowledge, and the whole history of each. At any rate, whatever any one else may have done in this way, it seems to me that new inferences remain to be drawn, of the nature of those which Plato drew: and those I here attempt to deduce and to illustrate.