" ... an object which exists for any time in its full perfection without any effect, is not the sole cause of that effect, but requires to be assisted by some other principle, which may forward its influence and operation. For as like effects necessarily follow from like causes, and in a contiguous time and place, their separation for a moment shows that these causes are not complete ones."—(I. p. 230.)
In addition to the bare notion of necessary connexion between the cause and its effect, we undoubtedly find in our minds the idea of something resident in the cause which, as we say, produces the effect, and we call this something Force, Power, or Energy. Hume explains Force and Power as the results of the association with inanimate causes of the feelings of endeavour or resistance which we experience, when our bodies give rise to, or resist, motion.
If I throw a ball, I have a sense of effort which ends when the ball leaves my hand; and if I catch a ball, I have a sense of resistance which comes to an end with the quiescence of the ball. In the former case, there is a strong suggestion of something having gone from myself into the ball; in the latter, of something having been received from the ball. Let any one hold a piece of iron near a strong magnet, and the feeling that the magnet endeavours to pull the iron one way in the same manner as he endeavours to pull it in the opposite direction, is very strong.
As Hume says:—
"No animal can put external bodies in motion without the sentiment of a nisus, or endeavour; and every animal has a sentiment or feeling from the stroke or blow of an external object that is in motion. These sensations, which are merely animal, and from which we can, a priori, draw no inference, we are apt to transfer to inanimate objects, and to suppose that they have some such feelings whenever they transfer or receive motion."—(IV. p. 91, note.)
It is obviously, however, an absurdity not less gross than that of supposing the sensation of warmth to exist in a fire, to imagine that the subjective sensation of effort or resistance in ourselves can be present in external objects, when they stand in the relation of causes to other objects.
To the argument, that we have a right to suppose the relation of cause and effect to contain something more than invariable succession, because, when we ourselves act as causes, or in volition, we are conscious of exerting power; Hume replies, that we know nothing of the feeling we call power except as effort or resistance; and that we have not the slightest means of knowing whether it has anything to do with the production of bodily motion or mental changes. And he points out, as Descartes and Spinoza had done before him, that when voluntary motion takes place, that which we will is not the immediate consequence of the act of volition, but something which is separated from it by a long chain of causes and effects. If the will is the cause of the movement of a limb, it can be so only in the sense that the guard who gives the order to go on, is the cause of the transport of a train from one station to another.
"We learn from anatomy, that the immediate object of power in voluntary motion is not the member itself which is moved, but certain muscles and nerves and animal spirits, and perhaps something still more minute and unknown, through which the motion is successively propagated, ere it reach the member itself, whose motion is the immediate object of volition. Can there be a more certain proof that the power by which the whole operation is performed, so far from being directly and fully known by an inward sentiment or consciousness, is to the last degree mysterious and unintelligible? Here the mind wills a certain event: Immediately another event, unknown to ourselves, and totally different from the one intended, is produced: This event produces another equally unknown: Till at last, through a long succession, the desired event is produced."—(IV. p. 78.)
A still stronger argument against ascribing an objective existence to force or power, on the strength of our supposed direct intuition of power in voluntary acts, may be urged from the unquestionable fact, that we do not know, and cannot know, that volition does cause corporeal motion; while there is a great deal to be said in favour of the view that it is no cause, but merely a concomitant of that motion. But the nature of volition will be more fitly considered hereafter.