Despite Nietzsche's conclusion that the known facts of existence do not bear it out, and the essential impossibility of discussing it to profit, the doctrine of eternal recurrence is by no means unthinkable. The celestial cycle put forward, as an hypothesis, by modern astronomy--the progression, that is, from gas to molten fluid, from fluid to solid, and from solid, by catastrophe, back to gas again--is easily conceivable, and it is easily conceivable, too, that the earth, which has passed through an uninhabitable state into a habitable state, may one day become uninhabitable again, and so keep seesawing back and forth through all eternity. But what will be the effect of eternal recurrence upon the superman? The tragedy of it, as we have seen, will merely serve to make him heroic. He will defy the universe and say "yes" to life. Putting aside all thought of conscious existence beyond the grave, he will seek to live as nearly as possible in exact accordance with those laws laid down for the evolution of sentient beings on earth when the cosmos was first set spinning. But how will he know when he has attained this end? How will he avoid going mad with doubts about his own knowledge? Nietzsche gave much thought, first and last, to this epistemological problem, and at different times he leaned toward different schools, but his writing, taken as a whole, indicates that the fruit of his meditations was a thorough-going empiricism. The superman, indeed, is an empiricist who differs from Bacon only in the infinitely greater range of his observation and experiment. He learns by bitter experience and he generalizes from this knowledge. An utter and unquestioning materialist, he knows nothing of mind except as a function of body. To him speculation seems vain and foolish: his concern is ever with imminent affairs. That is to say, he believes a thing to be true when his eyes, his ears, his nose and his hands tell him it is true. And in this he will be at one with all those men who are admittedly above the mass today. Reject empiricism and you reject at one stroke, the whole sum of human knowledge.