Dialogue Concerning the Two Chief World Systems

[Of Galileo.]

§ SALV. That is truly how it is. However, leaving behind the general contemplation of the whole, let us look at the consideration of the parts. Aristotle, in his first division, makes these two, and they are extremely diverse from one another, and in a certain way contrary; I mean the Celestial and the Elementary. The former is ungenerable, incorruptible, unalterable, impassible original: "ingenerabile, incorruttibile, inalterabile, impassibile"—these terms describe a heaven that is eternal and cannot be changed or affected by physical forces., etc. And the latter is exposed to a continuous alteration, mutation, etc. This difference he derives, as if from its original principle, from the diversity of local motions local motion: the movement of a body from one place to another; and he proceeds with this progression.

Stepping out, so to speak, from the Sensible World the physical world we perceive through our senses and retreating to the Ideal World, he begins architecturally to consider that, since nature is the principle of motion, it is fitting that natural bodies should be capable of local motion. He then declares local movements to be of three kinds: namely, circular, straight, and a mixture of the straight and the circular. The first two he calls "simple," because, of all lines, only the circular and the straight are simple. From here, narrowing his focus somewhat, he defines simple movements again: one is circular—that which is made around the center original: "mezzo"—referring to the center of the universe, which in the Aristotelian view is the center of the Earth.—and the others are straight, being upward and downward. Upward is that which moves away from the center; downward is that which goes toward the center. From this he infers, as is necessarily fitting, that all simple movements are restricted to these three species: namely, toward the center, away from the center, and around the center. This corresponds, he says, with a certain beautiful proportion to what was said above regarding the body: that it is perfected in three things, and so is its motion. Having established these movements, he continues by saying that since some natural bodies are simple and others are composed of them (and he calls simple bodies those that have a natural principle of motion, such as fire and Earth), it is fitting that simple movements belong to simple bodies, and mixed movements to compound bodies—in such a way, however, that the compound bodies follow the motion of the part that predominates in the composition.

§ SAGR. Please, Mr. Salviati, stop for a moment, because I feel so many doubts sprouting up from so many sides in this progression that I will be forced to either speak them if I want to listen attentively to the things you will add, or else withdraw my attention from what you are about to say if I want to keep the memory of these doubts.

§ SALV. I will very gladly stop, because I am running a similar risk myself, and I am on the verge of getting lost from one moment to the next while I must sail between rocks and such broken waves that they make me—