The dialectic theory, considered in its relation to physics, tended to substitute the study of uniformity for the study of mechanical causation. But the general conceptions established by science were a kind of soul in Nature; they were immaterial, they could not be perceived by sense, and yet, remaining as they did unchanged in a world of change, they were far truer, far more real, than the phenomena to which they gave unity and definition. Now these self-existent ideas, being subjective in their origin, readily reacted on mind, and communicated to it those attributes of fixedness and eternal duration which had in truth been borrowed by them from Nature, not by Nature from them. Plato argued that the soul was in possession of ideas too pure to have been derived from the suggestions of sense, and therefore traceable to the reminiscences of an ante-natal experience. But we can see that the reminiscence was all on the side of the ideas; it was they that betrayed their human origin by the birthmark of abstraction and finality—betokening the limitation of man’s faculties and the interest of his desires—which still clung to them when from a temporary law of thought they were erected into an everlasting law of things. As Comte would say, Plato was taking out of his conceptions what he had first put into them himself. And, if this consideration applies to all his reasonings on the subject of immortality, it applies especially to what he regards as the most convincing demonstration of any. There is one idea, he tells us, with which the soul is inseparably and essentially associated—namely, the idea of life. Without this, soul can no more be conceived than snow without cold or fire without heat; nor can death approach it without involving a logical contradiction. To assume that the soul is separable from the body, and that life is inseparable from the soul, was certainly an expeditious method of proof. To a modern, it would have the further disadvantage of proving too much. For, by parity of reasoning, every living thing must have an immortal soul, and every soul must have existed from all eternity. Plato frankly accepted both conclusions, and even incorporated them with his ethical system. He looked on the lower animals as so many stages in a progressive degradation to which human beings had descended through their own violence or sensuality, but from which it was possible for them to return after a certain period of penitence and probation. At other times he describes a hell, a purgatory, and a heaven, not unlike what we read of in Dante, without apparently being conscious of any inconsistency between the two representations. It was, indeed, an inconsistency such as we find in the highest order of intellects, the inconsistency of one who mediated between two worlds, between naturalistic metempsychosis on the one side, and ethical individualism on the other.