The next paragraph contains Hume's earliest[48] sketch of that critical inquiry into Human Nature on which he proposed to base all future philosophy. It is of course deeply interesting.
"'Tis evident that all the sciences have a relation, greater or less, to human nature; and that however wide any of them may seem to run from it, they still return back by one passage or another. Even Mathematics, Natural Philosophy, and Natural Religion, are in some measure dependent on the science of Man; since they lie under the cognizance of men, and are judged of by their powers and faculties.
"'Tis impossible to tell what changes and improvements we might make in these sciences were we thoroughly acquainted with the extent and force of human understanding, and could explain the nature of the ideas we employ, and of the operations we perform in our reasonings. And these improvements are the more to be hoped for in natural religion, as it is not content with instructing us in the nature of superior powers, but carries its views further, to their disposition towards us, and our duties towards them; and consequently we ourselves are not only the beings that reason, but also one of the objects concerning which we reason."
"If, therefore, the sciences of mathematics, natural philosophy, and natural religion, have such a dependence on the knowledge of man, what may be expected in the other sciences, whose connection with human nature is more close and intimate?... In these four sciences of Logic, Morals, Criticism, and Politics, is comprehended almost everything which it can any way import us to be acquainted with, or which can tend either to the improvement or ornament of the human mind.
"Here then is the only expedient, from which we can hope for success in our philosophical researches, to leave the tedious lingering method, which we have hitherto followed, and instead of taking now and then a castle or village on the frontier, to march up directly to the capital or centre of these sciences, to human nature itself; which being once masters of, we may everywhere else hope for an easy victory. From this station we may extend our conquests over all those sciences which more intimately concern human life, and may afterwards proceed at leisure to discover more fully those which are the objects of pure curiosity. There is no question of importance, whose decision is not comprised in the science of man; and there is none, which can be decided with any certainty, before we become acquainted with that science. In pretending, therefore, to explain the principles of human nature, we in effect propose a complete system of the sciences, built on a foundation almost entirely new, and the only one upon which they can stand with any security."[49] Ibid. pp. 13-14.
The present writer has a special interest in citing these passages, because they do in fact defend as well as describe the procedure of his very next chapter.
Such then at an early age was Hume's keen-edged critical appreciation of those intellectual conditions required for a Philosophy of the Sciences, or as he calls it, the "true Metaphysics." In order to supplement his clever and clear idea by a very practical delineation of the metaphysical territory, we turn to another great thinker, the founder of our modern natural science, the great Lord Verulam.[50]
Bacon divides Philosophy according to its objects, which are three,—God, Nature, Man. Take, then, Natural Philosophy; it is well said that the truth of nature lies deeply hidden, and it is also well said that the Producer imitates Nature. Natural Philosophy divides itself accordingly into the inquisition of causes and the production of effects; it is both speculative and operative. There is indeed an intercourse between causes and effects, and both these kinds of knowledge. All true and fruitful Natural Philosophy has a double scale or ladder,—ascendent and descendent; ascending from experiment to first causes; descending thence to fresh experiment and always fresh productiveness.[51]
The ascending half is divided into two moieties, of which one is the science of Physics, the other of Metaphysics. In distinguishing these two, Bacon so far agrees with antiquity as to say,—"That Physic supposes in nature only a being and moving and natural necessity; whereas Metaphysic supposes also a Mind and Idea. For that which I shall say comes perhaps to this."[52] Or, to put it in another light, he writes elsewhere:—"Physique, taking it according to the derivation, and not according to our idiom for medicine, is situate in a middle term or distance between natural history and Metaphysique. For natural history describeth the variety of things; Physique, the causes, but variable or respective causes; and Metaphysique, the fixed and constant causes."[53]