BOOK TWO.

CHAPTER V.

On local motion and its definition.

Local motion, which the Philosopher Referring to Aristotle defines in Book 5 of the Physics, text 1 and following, is the translation from place to place. This motion is the first and most noble of all others, as Aristotle proves in Book 8 of the Physics, text 10 and following, because it is the cause and foundation of all other motions.

Local motion is twofold: it is either rectilinear or circular. Rectilinear is that which takes place along a straight line; circular is that which takes place along a circular line. Both motions, the rectilinear as well as the circular, have their causes and principles by which they are governed and directed.

Aristot. 8. Phys. text. 12.

Circular motion is the simplest and most uniform, because in it the moving object always remains at the same distance from the center and always moves around the same center. Rectilinear motion, however, is more complex and variable, because in it the moving object is always changing its distance from the center and from the limits of the motion.

From this it follows that circular motion is natural to celestial bodies, which are incorruptible and eternal, whereas rectilinear motion is natural to elementary bodies, which are corruptible and mutable.