"We are like a man in a boat sailing with the stream, who does not know that the water is flowing, and who cannot see the banks: how is he to discover whether the boat is moving?" Cusanus, in fact, denies the fundamental Aristotelian dogma that the earth is the central point of the universe, because, on general grounds, there _can_ be no absolute central point. This gave a shock to the "geocentric theory" from which it never recovered. Worse shocks, however, were to come. The name of the man who actually (as Luther complained) turned the world upside down, is notorious enough. Poles and Germans alike have claimed the nationality of Nicolaus Copernicus (1473-1543); who, having been a student at Cracow and in Italy, became a prebendary in Frauenburg Cathedral. THE NEW ASTRONOMY.--The general criticisms of Cusanus were elaborated by Copernicus. The senses cannot inform us (when any motion takes place) _what_ it is that moves. It may be the thing perceived that moves, or the percipient--or both. And it would be _possible_ to account for the movements of celestial bodies by the supposition that it is the earth that moves, and not they. Copernicus' whole work consisted in the mathematical demonstration that this hypothesis could account for the phenomena as we observe them. In fact, when these demonstrations were eventually published (it was only on his death-bed that Copernicus received a copy of his book--and he had already lost consciousness) they were introduced by a discreet preface, which intimated that the whole thing might safely be regarded as a _jeu d'esprit_ on the part of an eccentric mathematician. And this editorial _caveto_, though written by another hand, preserved the Copernican theories from the notoriety that might otherwise have attended, and afterwards did attend, them. Copernican conceptions were semi-traditional. The sun displaces the earth as the central point of the universe: around it revolve the planets--including the earth; and, at an immeasurable distance, is the immovable heaven of the fixed stars. Copernicus left it an open question whether or no the universe was infinite. It remained for his successor, the greatest of the Renaissance thinkers, Giordano Bruno (1548-1600) to declare it to be limitless, and to contain an infinity of worlds like our own. The fixed stars became, for him, suns surrounded by planets. The traditional distinction between the celestial and sublunary spheres had vanished. The bewilderment and indignation excited by these ideas, revolting to the conscience of his time, cost their author his life.