And again, "It is true, the explanation of one set of facts may be of the same nature as the explanation of the other class; but then, that the cause explains both classes, gives it a very different claim upon our attention and assent from that which it would have if it explained one class only. The very circumstance that the two explanations coincide, is a most weighty presumption in their favour. It is the testimony of two witnesses in behalf of the hypothesis; and in proportion as these two witnesses are separate and independent, the conviction produced by their agreement is more and more complete. When the explanation of two kinds of phenomena, distinct, and not apparently connected, leads us to the same cause, such a coincidence does give a reality to the cause, which it has not while it merely accounts for those appearances which suggested the supposition. This coincidence of propositions inferred from separate classes of facts, is exactly what we noticed in the last book, as one of the most decisive characteristics of a true theory, under the name of Consilience of Inductions.
"That Newton's first rule of philosophizing, so understood, authorizes the inferences which he himself made, is really the ground on which they are so firmly believed by philosophers. Thus, when the doctrine of a gravity varying inversely as the square of the distance from the body, accounted at the same time for the relations of times and distances in the planetary orbits and for the amount of the moon's deflection from the tangent of her orbit, such a doctrine became most convincing: or, again, when the doctrine of the universal gravitation of all parts of matter, which explained so admirably the inequalities of the moon's motions, also gave a satisfactory account of a phenomenon utterly different—the precession of the equinoxes. And of the same kind is the evidence in favour of the undulatory theory of light, when the assumption of the length of an undulation, to which we are led by the colours of thin plates, is found to be identical with that length which explains the phenomena of diffraction; or when the hypothesis of transverse vibrations, suggested by the facts of polarization, explains also the laws of double refraction. When such a convergence of two trains of induction points to the same spot, we can no longer suspect that we are wrong. Such an accumulation of proof really persuades us that we have to do with a vera causa. And if this kind of proof be multiplied,—if we again find other facts of a sort uncontemplated in framing our hypothesis, but yet clearly accounted for when we have adopted the supposition,—we are still further confirmed in our belief, and by such accumulation of proof we may be so far satisfied as to believe without conceiving it possible to doubt. In this case, when the validity of the opinion adopted by us has been repeatedly confirmed by its sufficiency in unforeseen cases, so that all doubt is removed and forgotten, the theoretical cause takes its place among the realities of the world, and becomes a true cause." (Ibid. B. XII., chap. xiii., art. 10.)
The reader of this Essay will be pleased to remark as he proceeds that its argument is made up of a diversity of proofs (very many among them being inductive), and that they all lend each other mutual support and become consilient at last.
[CHAPTER II.]
PHILOSOPHY OF DESIGN.
"It is an assured truth, and a conclusion of experience, that a little or superficial knowledge of Philosophy may incline the mind of Man to Atheism, but a farther proceeding therein doth bring the mind back again to Religion: for in the entrance of Philosophy, when the second causes, which are next unto the senses, do offer themselves to the mind of man, if it dwell and stay there it may induce some oblivion of the highest Cause; but when a man passeth on farther, and seeth the dependence of causes and the works of Providence; then, according to the allegory of the poets, he will easily believe that the highest link of Nature's chain must needs be tied to the foot of Jupiter's chair."
Lord Bacon's Advancement of Learning, Book I.
"Deus sine dominio, providentiâ, et causis finalibus nihil aliud est quam fatum et natura. A cæcâ necessitate metaphysicâ, quæ eadem est et semper et ubique, nulla oritur variatio. Tota rerum conditarum pro locis ac temporibus diversitas, ab ideis et voluntate entis necessario existentis solummodo oriri potuit."—Sir Isaac Newton, Scholium at close of Principia.
"Tax not my sloth that I
Fold my arms beside the brook;
Each cloud that floated in the sky
Writes a letter in my book.
"There was never mystery
But 'tis figured in the flowers;
Was never secret history
But birds tell it in the bowers."
Emerson's Poems.—The Apology.