We live in a world in which effect follows cause in an orderly and, we are apt to suppose, invariable rhythm. According to modern modes of thought, it matters not where we tap the fount of scientific inspiration, we always find that the untiring search for the antecedents of any event is founded on the conviction that for that event there is some ascertainable cause. Even chance has yielded to the statistical method, so that its laws may be formulated. By dealing with larger and larger numbers we eliminate more and more the idiosyncrasies of the particular case. And thus we come to realise that what we call chance in the tossing of a coin is only our ignorance of the nature and immediate cause of these idiosyncrasies. Just in so far as our science or its application is imperfect, do we project upon the screen of nature, woven by our experience, the shadow of fortuity, blurring the details of processes which, to less imperfect mental vision, would stand out clearly as causally related. Thus it arises that, for those who have been led to this point of view, the doctrine of evolution, as applicable throughout the range of an experience which science indefinitely prolongs, gives expression to the daily strengthening belief that the state of matters at any given moment is the outcome of a state of matters in the preceding moment, and in like manner serves to determine the state of matters in the moment that follows.
As to the origin of our belief in the universality of causation there has been much discussion, one school of thought contending that it is the outcome of experience, another school of thought urging that it is prior to and a condition of all scientific interpretation. "It is commonly urged," said Huxley, "that the axiom of causation cannot be derived from experience, because experience only proves that many things have causes, whereas the axiom declares that all things have causes. The syllogism, 'Many things which come into existence have causes, A has come into existence, therefore A had a cause' is obviously fallacious, if A is not previously shown to be one of the 'many things.' And this objection is perfectly sound so far as it goes. The axiom of causation cannot possibly be deduced from any general proposition which simply embodies experience. But it does not follow that the belief, or expectation, expressed by the axiom is not a product of experience generated antecedently to, and altogether independently of, the logically unjustifiable language in which it is expressed. In fact,"he continues," the axiom of causation resembles all other beliefs of expectation in being the verbal symbol of a purely automatic act of the mind, which is altogether extra-logical, and would be illogical, if it were not constantly verified by experience."In other words, in this as in other matters where the schematic constructions of science are concerned, we sweep a curve of interpretation through a series of observed phenomena and carry it on beyond the confines of observation to its ideal limits.
Yet further discussion has arisen as to the nature of the links which join cause to effect. And as this will serve to emphasise the distinction between scientific and metaphysical causation, we may look into the question a little further.
Glanvill, in his Scepsis Scientific a, published in 1665 , says: "All knowledge of causes is deductive; for we know of none by simple intuition, but through the mediation of their effects. So that we cannot conclude anything to be the cause of another but from its continual accompanying it, for the causality itself is insensible." Let us specially note these words "for the causality itself is insensible." "What we call experience," said Hobbes a few years before, "is nothing else but remembrance of what antecedents have been followed by what consequents." Such statements as these may have been the seeds which germinated in the mind of Hume and developed into his well-known theory of causation. In any case it is evident that he thought the matter out for himself with his customary vigour and independence. We may profitably make his treatment of the subject our starting-point.
"When we look about us towards external objects, and consider the operation of causes," said Hume in that section of the Enquiry which treats of the Idea of Necessary Connection, "we are never able in a single instance to discover any power or necessary connection, any quality, which binds the effect to the cause, and renders the one an infallible consequence of the other. We only find that the one does actually in fact follow the other. The impulse of one billiard ball is attended with motion in the second. This is the whole that appears to the outward senses. The mind feels no sentiment or inward impression from this succession of objects. Consequently there is not, in any single, particular instance of cause and effect, anything which can suggest the idea of power or necessary connection".
For a comprehension of Hume's conception stress must be laid, in this passage, on the words "in a single instance." When he says that we are never able in a single instance to discover any power or necessary connection, these four words are not merely inserted to emphasise the never; they are to be taken literally. We are never able, from the study of a single and isolated case or example, to discover any power or necessary connection. According to Hume:
This idea of a necessary connection amongst events arises from a number of similar instances, which occur, of the constant conjunction of these events; nor can that idea ever be suggested by any one of these instances, surveyed in all possible lights and positions. But there is nothing in a number of instances, different from every single instance, which is supposed to be exactly similar; except only, that after a repetition of similar instances the mind is carried by habit, upon the appearance of the one event, to expect its usual attendant, and to believe that it will exist. This connection, therefore, which we feel in the mind, or customary transition of the imagination from one object to its usual attendant, is the sentiment or impression from which we form the idea of power or necessary connection. The first time a man saw communication of motion by impulse, as by the shock of two billiard balls, he could not pronounce that the one event was connected; but only that it was conjoined with the other. After he has observed several instances of this nature, he then pronounces them to be connected. What alteration has happened to give rise to this new idea of connection? Nothing but that he now feels these events to be connected in his imagination, and can readily foretell the existence of the one from the appearance of the other. When many uniform instances appear, and the same object is always followed by the same event, we then begin to entertain the notion of cause or connection.