Note 43 — Chapter I · THE CONTRIBUTION OF THE ANCIENT WORLD

Aristotle’s abstruse reasoning about the immobility of the earth is interpreted by Duhem, Système, vol. i, pp. 219–230. Duhem clarifies the arguments of the Stagirite by resolving them into four main propositions:

(1) “The movement of the heavens requires the existence of an immovable body distinct from the heavens at the center of the universe” (Duhem, Système, vol. i, p. 220). Why such an immovable body is necessary is explained in De caelo, I, 8, and in Physics, IV, 4 (cited by Duhem, Système, vol. i, pp. 198–210, 221). Later writers and commentators confused Aristotle’s views here set forth with a theory which the philosopher—if he wrote it—presents in the De motu animalium and which is, in brief, as follows. “For every animal that moves there must be without it something immovable, but supporting itself upon which that which is moved moves. For were that something always to give way (as it does for mice walking in grain, or persons walking in sand) advance would be impossible, and neither would there be any walking unless the ground were to remain still” (De motu anim., 2; translated by A. S. L. Farquharson in the Works of Aristotle, 1913, p. 698b). Although the writer of this passage expressly states that he does not intend this simple theory to be applied to the movements of the heaven in relation to the earth, it was, none the less, passed on by way of the Moslems to the West as an argument in favor of the immobility of the earth.

(2) “Physical reasons prove that it is not possible for the earth to move” with a circular motion. The normal motion of the particles which compose the earth is in a straight line toward the earth’s center. Correspondingly “the movement which is natural to each part must also be natural to the whole, in such a way that the earth taken as a whole certainly has for its natural motion that movement in a straight line and directed toward the center which characterizes heavy bodies” (Duhem, Système, vol. i, p. 226). Any other movement, such as a movement of rotation, “being, then, constrained and unnatural ... could not be eternal. But the order of the universe is eternal” (De caelo, II, 14; translated by J. L. Stocks in the Works of Aristotle, 1922, p. 296a).

(3) “Experiments show that as a matter of fact the earth does not move at all.” If the earth moved “there would have to be passings and turnings of the fixed stars. Yet no such thing is observed” (De caelo, II, 14; Stocks’s translation, p. 296b). In other words, if the earth moved one would expect to observe parallaxes of the fixed stars (Duhem, Système, vol. i, p. 227). “It is clear, then, that the earth must be at the center and immovable, not only for the reasons already given, but also because 371heavy bodies thrown quite straight upward return to the point from which they started, even if they are thrown to an infinite distance” (De caelo, II, 14; Stocks’s translation, p. 296b).

(4) “Physics teaches us the cause of the immobility of the earth.” As all heavy bodies tend to seek the center of the universe, the various parts of the earth have arranged themselves around the center in such a manner that an equilibrium is established, and this equilibrium produces immobility (De caelo, II, 14, Stocks’s translation, p. 297a; Duhem, Système, vol. i, pp. 216, 228–229).