Since Kant explained space and time to be merely "forms of perception"--space the form of external, time of internal, sensitivity--there has been a keen controversy, which still continues, over this important problem. A large section of modern metaphysicians have persuaded themselves that this "critical fact" possesses a great importance as the starting-point of "a purely idealist theory of knowledge," and that, consequently, the natural opinion of the ordinary healthy mind as to the _reality_ of time and space is swept aside. This narrow and ultra-idealist conception of time and space has become a prolific source of error. It overlooks the fact that Kant only touched one side of the problem, the _subjective_ side, in that theory, and recognized the equal validity of its _objective_ side. "Time and space," he said, "have empirical reality, but transcendental ideality." Our modern monism is quite compatible with this thesis of Kant's, but not with the one-sided exaggeration of the subjective aspect of the problem; the latter leads logically to the absurd idealism that culminates in Berkeley's thesis, "Bodies are but ideas; their essence is in their perception." The thesis should be read thus: "Bodies are only ideas for my personal consciousness; their existence is just as real as that of my organs of thought, the ganglionic cells in the gray bed of my brain, which receive the impress of bodies on my sense-organs and form those ideas by association of the impressions." It is just as easy to doubt or to deny the reality of my own consciousness as to doubt that of time and space. In the delirium of fever, in hallucinations, in dreams, and in double-consciousness, I take ideas to be true which are merely fancies. I mistake my own personality for another (_vide_ p. 185); Descartes' famous _Cogito ergo sum_ applies no longer. On the other hand, the reality of time and space is now fully established by that expansion of our philosophy which we owe to the law of substance and to our monistic cosmogony. When we have happily got rid of the untenable idea of "empty space," there remains as the infinite "space-filling"-medium matter, in its two forms of ether and mass. So also we find a "time-filling" event in the eternal movement, or genetic energy, which reveals itself in the uninterrupted evolution of substance, in the _perpetuum mobile_ of the universe.