Dialogue Concerning the Two Chief World Systems

...no, as they say, lose one’s bearings; however, before adding more to the pile, propose your difficulties.

SAGR. You, along with Aristotle, at the beginning separated me somewhat from the Sensible World the physical world perceived by the senses to point out the architecture with which it was to be built; and to my delight, you began by telling me that a natural body is by nature mobile, since nature has been defined elsewhere as the principle of motion. At this point, a small doubt arose in me. Why did Aristotle not say that among natural bodies, some are mobile by nature and others are immobile,

The definition of nature is either defective or introduced out of place by Aristotle.

given that in the definition, nature is called the principle of motion and of rest? For if natural bodies all possess a principle of movement, then either it was not necessary to include "rest" in the definition of nature, or it was not necessary to introduce such a definition in this place. Furthermore, as for declaring to me which movements he intends to be "simple," and how he determines them by spaces—calling simple those which are made along simple lines, which are the circular and the straight only—I accept this quietly. I do not even care to sharpen against him the objection

A helix around a cylinder could be called a simple line.

of the helix around a cylinder a spiral shape like a screw thread, which, because it is similar to itself in every one of its parts, seems as though it could be numbered among the simple lines. But I do feel somewhat resentful upon hearing him restrict himself (while he seems to want to repeat the same definitions with other words) to calling the one "motion around the center" and the other "upward and downward" original Latin: "sursum, & deorsum"; these terms are not used outside of a world already constructed, but they suppose it not only to be constructed, but already inhabited by us. For if straight motion is simple because of the simplicity of the straight line, and if simple motion is natural, then let it be made in whatever direction you please—I mean upward, downward, forward, backward, to the right, and to the left, and any other difference one can imagine—provided it is straight, it must suit some simple natural body; or if not, Aristotle's assumption is lacking. It is seen, moreover, that Aristotle hints that there is only one circular motion in the world, and

Aristotle fits the rules of architecture to the construction of the World, and not the construction to the rules.

consequently only one center, to which alone the straight motions of "up" and "down" refer. These are all signs that he intends to "shuffle the cards in our hands" an idiom meaning to deceive or change the terms of the argument mid-stream, and that he wants to fit the architecture to the building, rather than constructing the building according to the rules of architecture. For if I say that in the universality of nature there can be thousands of circular motions, and consequently thousands of centers, there will also be thousands of motions upward and...