In a recent paper the writer had alluded to Professor Huxley's theory of the mental process of abstraction—viz., that it was analogous to the physical process of taking compound photographs; that, accordingly, the vague representations of men, hills, and rivers in dreams might rightly be described as generic-and had maintained that this theory could not stand, because a general conception must cover contradictories, and contradictories could not be represented in one image. The question had been threshed out 200 years ago. Locke had alluded to the general idea of a triangle as one that “must neither be oblique nor rectangle, neither equilateral, equicrural, nor scalenon, but all and none of these at once.” On this Bishop Berkeley had taken him to task in his gravely sarcastic fashion, observing that if any one could frame such an idea as this of a triangle “he would be sorry to dispute him out of it.” The difficulty had not escaped Kant. Its solution, indeed, formed an important feature in his Philosophy.“No image,” he observes, “could ever be adequate to our conception of a triangle in general.” He was of opinion, therefore, that not images, but what he calls schemata, lie at the foundation of general conceptions. The schema is a sort of mental rule for the construction of a triangle, and is a product of thought as distinguished from reproductive imagination simply. The distinction was all-important. The two faculties were often in inverse proportion to one another. This radical error was the source of further error, in connection with the doctrines of necessary truth and causation. In Professor Huxley's view, the reminiscence “I was in pain yesterday,” might “properly be said to be necessary.” If that was so, the distinction between necessary certainty and ordinary certainty was wholly illusory; and, in that case, nearly all that had been called philosophy, from Plato to Hume, was idle words. The truth, however, was far otherwise. After some further argument and illustration, intended to bring out the writer's view of the character of necessary truth, he went on to say that Professor Huxley divided so-called necessary truths into two classes-(1) Identical propositions; (2) Truths of experience. Identical propositions, such as “A is A,” depended on the possibility of intelligible speech. This took it for granted that it was the easiest thing in the world to say what was an identical proposition, and what was not. If we thought it out, however, it did not seem to be so. “Black is black” is an identical proposition, no doubt. What about “Black and white in alternate patches are piebald”? That was also, perhaps, identical. What about “Blue and yellow mixed are green”? That was certainly not identical, yet it stood on a different footing from a mere truth of experience, as we could see the blue and yellow in the green-that is, the whole cause in the effect. This seemed to him to make very clear the inadequacy of the famous Humist
doctrine of causation, that “difference” and “constant conjunction” between two phenomena “are all the circumstances that enter into the idea of cause and effect.” The truth rather was that we never wholly understood the causal connection between two phenomena till we perceived the identity between the cause and the effect. In illustration of this he cited a passage from Spinoza on the efficient cause of a circle.