tween cause and effect which remains entirely unaffected by the Hume- Kant- Lotzian arguments.
It is doubtless true that the bare notion of causality contains no suggestion of such an equivalence. For all that, the demonstration of quantitative relations between successive events gives us a far more perfect understanding of their connection than the mere knowledge that they stand related as cause and effect. Once the possibility of such an understanding is suggested, it must, however seldom realizable, remain ever after the beau-ideal of the physicist. If, however, investigation proves it to be realizable in the great majority of cases, and this, I take it, is the actual fact, a strong presumption is raised with regard to any particular physical event that it can be shown to be connected with previous physical events by relations of quantitative exactitude.
Now the form in which this ideal has been realized is this, that the amount of energy contained in the cause has been found to be equal to the amount contained in the effect. But if the cause always contains the same amount of energy as the effect, the sum of all the causes in the universe must contain the same amount of energy as the sum of all their effects, and the quantity of energy in existence must be constant — which is the law of the conservation of energy.[1]
If now from the causal relations of physical events inter se we turn to those between physical and mental events, it is evident that nothing resembling physical explanation is here possible. There can be no exact quantitative relation between cause and effect, for the two are incommensurable. We cannot picture to ourselves the cause passing over into the effect, for they belong to different orders of existence. "The passage from the physics of the brain to the facts of consciousness is unthinkable." The cause belonging to a given effect can at most be indicated, but the connection between the two cannot be explained.
Of course it is by no means inconceivable that a certain event in the nervous centres should be the indispensable condition of a sensation, or that a volition should be the indispensable condition of a nervous discharge into the muscles. But no such phenomenal bond can exist as that which connects two physical events, and we can only say that the cause is invariably succeeded by the effect. To say that the nervous event actively produced the sensation, or that the volition actively produced the motor discharge, would be to assume the existence of precisely that unseen bond which Hume, Kant, and Lotze have disproved.
We may correctly say that a physical cause produces a physical effect; the fall of a stone, for example, being a mode of motion, may as properly be said to produce that other mode of motion which we call heat,
- ↑ Wundt, Die physikalischen Axiome, Erlangen, 1866, pp. 128-132.
