All this is ridiculous physics enough; but Plato knew--though he forgot sometimes--that his physics were playful. What it is important for us now to remember is rather that, under this childish or metaphorical physics, there is a serious morality. After all, the _use_ of opium is that it is a narcotic; no matter why, physically, it is one. The _use_ of the body _is_ the mind, whatever the origin of the body may be. And it seems to dignify and vindicate these uses to say that they are the “causes” of the organs that make them possible. What is true of particular organs or substances is true of the whole frame of nature. Its _use_ is to serve the good--to make life, happiness, and virtue possible. Therefore, speaking in parables, Plato says with his whole school: Discover the right principle of action, and you will have discovered the ruling force in the universe. Evoke in your rapt aspiration the essence of a supreme good, and you will have understood why the spheres revolve, why the earth is fertile, and why mankind suffers and exists. Observation must yield to dialectic; political art must yield to aspiration. It took many hundred years for the revolution to work itself out; Plato had a prophetic genius, and looked away from what he was (for he was a Greek) to what mankind was to become in the next cycle of civilization. In Dante the revolution is complete, not merely intellectually (for it had been completed intellectually long before, in the Neoplatonists and the Fathers of the Church), but complete morally and poetically, in that all the habits of the mind and all the sanctions of public life had been assimilated to it. There had been time to reinterpret everything, obliterating the natural lines of cleavage in the world, and substituting moral lines of cleavage for them. Nature was a compound of ideal purposes and inert matter. Life was a conflict between sin and grace. The environment was a battle-ground between a host of angels and a legion of demons. The better and the worse had actually become, as Socrates desired, the sole principles of understanding.