of ideas as a physical world; but it is one of a peculiar nature, which is not, like the other, accessible to all, and is subject to its own laws, which are laws of association. By these very different characteristics, it separates itself so sharply from the outer world that all endeavour to bring the two together seems shocking; and it is very easy to understand that many minds should wish to remain faithful to the conception that ideas form a mental or moral world. No metaphysical reasoning could prevail against this sentiment, and we must give up the idea of destroying it. But we think we have shown that idea, like sensation, comprises at the same time the physical and the mental.
