Definition of motion and its division. Motion, being made by two different powers, is said to be of two natures, and therefore divides into two parts, of which one is spiritual and the other material. Spiritual, among us, is that which is made by the power of the imagination; material is that made by material bodies. Division of motion. The motion made by bodies through material instruments likewise divides into two parts, of which one is primitive and the other derivative. Nature of the primitive and the derivative. Primitive is called that thing which by its own motion casts out of itself the thing moved. It is impossible that derivative motion, made upward or along a level line, should ever equal its own motion in the derivative. Always the time and the derivative motion will be in proportion with the time and the primitive motion. Example. Let then a transverse motion be made by the primitive motion, of brief quantity: I say that the derivative motion too will be brief, and will bear a certain proportion to the primitive. If again the primitive motion be doubled in time and in length, I say the like will be done by the time and the derivative motion. Experience. If you draw the bowstring by a very small part and then release it free with its arrow, certainly the arrow's motion, once it parts from that of the string, will itself be short and brief. And the like you will find in a short blowpipe, like a sewing-ring, and in the ball set on the mouth of the bombard, and other such: being little pushed, they drive themselves little forward. On circular motions. That same which is said above will be found in circular movements. But, as said, it will not be found that outside the natural motions there is any derivative movement that surpasses the swiftness of the primitive.

By primitive motion I mean that which is made by the moved thing as it moves with the motion of the cause that moves it, while the two touch together; and when their separation is caused, then derivative motion is born. Here one must show that this primitive motion is not ended with the weariness of its cause, but rather lasts to the middle of its course.