will be their total effect. We next satisfy ourselves as to whether the actual movement exactly coincides with the movement foretold; and if this is so, we attribute it to the causes from which we have deduced it. Thus, in order to discover the causes of the planetary motions, we seek by simple induction the laws of two causes: first, the force of primitive impulsion in the direction of the tangent; next, an accelerative attracting force. From these inductive laws we deduce by calculation the motion of a body submitted to their combined influence; and satisfying ourselves that the planetary motions observed coincide exactly with the predicted movements, we conclude that the two forces in question are actually the causes of the planetary motions. "To the Deductive Method," says Mill," the human mind is indebted for its most conspicuous triumphs in the investigation of nature. To it we owe all the theories by which vast and complicated phenomena are embraced under a few simple laws." Our deviations have led us further than the direct path; we have derived efficiency from imperfection. X. If we now compare the two methods, their aptness, function, and provinces, we shall find, as in an abstract, the history, divisions, hopes, and limits of human science. The first appears at the beginning, the second at the end. The first necessarily gained ascendency in Bacon's time,1 and now begins to lose it; the second necessarily lost ascendency in Bacon's time, and now begins to regain it. So that science, after having 1 Mill's Logic, i. 526. passed from the deductive to the experimental state, is now passing from the experimental to the deductive. Induction has for its province phenomena which are capable of being decomposed, and on which we can experiment. Deduction has for its province indecomposable phenomena, or those on which we cannot experiment. The first is efficacious in physics, chemistry, zoology, and botany, in the earlier stages of every science, and also whenever phenomena are but slightly complicated, within our reach, capable of being modified by means at our disposal. The second is efficacious in astronomy, in the higher branches of physics, in physiology, history, in the higher grades of every science, whenever phenomena are very complicated, as in animal and social life, or lie beyond our reach, as the motions of the heavenly bodies and the changes of the atmosphere. When the proper method is not employed, science is at a stand-still: when it is employed, science progresses. Here lies the whole secret of its past and its present. If the physical sciences remained stationary till the time of Bacon, it was because men used deduction when they should have used induction. If physiology and the moral sciences are now making slow progress, it is because we employ induction when deduction should be used. It is by deduction, and according to physical and chemical laws, that we shall be enabled to explain physiological phenomena. It is by deduction, and according to mental laws, that we shall be enabled to explain historical phenomena.1 And that which has 1 See chapter 9, book vi. v. 2, 478, on The Physical or Concrete Deductive Method as applied to Sociology; and chapter 13, book iii., for explanations, after Liebig, of Decomposition, Respiration, the Action of Poisons, etc. A whole book is devoted to the logic of the moral sciences; I know no better treatise on the subject. |