age. But he seems to have had a measure of sympathy with the religious movement, and he fell into discredit during his latter years on account of his liberal, humanistic tendency. He began the elaboration of his astronomical theory already in 1506, but he was hesitant about its publication, and the first printed copy of his work De revolutionibus orbium coelestium only appeared shortly before his death (1543). The matter which specially concerns us is the epistemological presuppositions which form the basis of this work. Two of its presuppositions must claim our attention.
Nature always takes the simplest course. The theory of the whole universe revolving around so small a body as the earth is inconsistent with this principle. And the case is similar with the theory that the planetary orbits should not be simple circles but a very complicated system of epicycles. On the other hand, if we regard the sun as the center of the universe, and the earth and the planets as revolving around it, we have a very simple theory of the universe.
The second presupposition is the principle of the relativity of motion previously suggested by Cusanus. The perception of motion is not adequately explained by the mere reference to the fact that a perceived object has really changed its position in space. It may likewise be due to the fact that the perceiving subject has moved. If we therefore assume that the earth, from which we observe the motion of the heavenly bodies, is itself in motion (around its axis and around the sun), we will be in position to explain the phenomenon quite as well (only more simply) as the traditional theory.
Copernicus still adhered to the idea of a finite universe and regarded the firmament of the fixed stars, the boun-
