That necessary truth is progressive;—that science is the idealization of facts, and that this process goes on from age to age, and advances with the advance of scientific discovery;—these are doctrines which I have endeavoured to establish and to elucidate. If these doctrines are true, they are so important that I may be excused should I return to them again and again, and trace their consequences in various directions. Especially I would examine the bearing of these doctrines upon our religious philosophy. I have hitherto abstained in a great measure from discussing religious doctrines; but such a reserve carried too far must deprive our philosophy of all completeness. No philosophy of science can be complete which is not also a philosophy of the universe; and no philosophy of the universe can satisfy thoughtful men, which does not include a reference to the power by which the universe came to be what it is. Supposing, then, such a reference to be admitted, let us see what aspect our doctrines give to it.
1. (How can there be necessary truths concerning the actual universe?)—In looking at the bearing of our doctrine on the philosophy of the universe, we are met by a difficulty, which is indeed, only a former difficulty under a new aspect. When we are come to the conclusion that science consists of facts idealized, we are led to ask, How this can be? How can facts be idealized? How can that which is a fact of external observation become a result of internal thought? How can that which was known à posteriori become known à priori? How can the world of things be identified with the world of thoughts? How can we discover a necessary connexion among mere phenomena?
Or to put the matter otherwise: How is it that the deductions of the intellect are verified in the world of sense? How is it that the truths of science obtained à priori are exemplified in the general rules of facts observed à posteriori? How is it that facts, in science, always do correspond to our ideas?
I have propounded this paradox in various forms, because I wish it to be seen that it is, at first sight, a real, not merely a verbal contradiction, or at least a difficulty. If we can discover the solution of this difficulty in any one form, probably we can transpose the answer so as to suit the other forms of the question.
2. Suppose the case to be as I have stated it; that in some sciences at least, laws which were at first facts of observation come to be seen as necessary truths; and let us see to what this amounts in the several sciences.
It amounts to this: the truths of Geometry, such as we discern them by the exercise of our own thoughts, are always verified in the world of observation. The laws of space, derived from our Ideas, are universally true in the external world.
In the same way, as to number: the laws or truths respecting number, which are deduced from our Idea of Number, are universally true in the external world.
In the same way, as to the science which deals with matter and force: the truths of which I have spoken as derived from Ideas:—that action is equal to reaction; and that causes are measured by their effects;—are universally verified in all the laws of phenomena of the external world, which are disclosed by the science of Mechanics.
In the same way with regard to the composition and resolution of bodies into their elements; the truths derived from our Idea of Matter:—that no composition or resolution can increase or diminish the quantity of matter in the world, and that the properties of compounds are determined by their composition;—are truths derived from Ideas of quantity of matter, and of composition and resolution; but these truths are universally verified when we come to the facts of Chemistry.
In the same way it is a truth flowing from the Ideas of the Kinds of things, (as the possible subject of general propositions expressed in language,) that the kinds of things must be definite; and this law is verified whenever we express general propositions in general terms: for instance, when we distinguish species in Mineralogy.