This is a philosophy book. The more-crude theories of early times are to be chiefly distinguished from the less-crude theories of to-day as being largely the products of random guesswork. Hypothesis, or guesswork, indeed, lies at the foundation of all scientific knowledge. The riddle of the universe, like less important riddles, is unravelled only by approximative trials, and the most brilliant discoverers have usually been the bravest guessers. Kepler's laws were the result of indefatigable guessing, and so, in a somewhat different sense, was the wave-theory of light. But the guesswork of scientific inquirers is very different now from what it was in older times.