Hitherto Unpublished Works, Fascicle 4

There is something that is contrary to all of them in species, yet similar in genus, or else the world would be incomplete. This 94 b 2. is seen more clearly through the combination of the four qualities. They can be combined in four ways: earth is called cold and dry, water is cold and wet, air is wet and hot, and fire is hot and dry. Therefore, if one of these exists with its qualities, the others must also exist.

In the same way, the absolutely heavy grave absolute: a body that always moves toward the center of the universe, regardless of where it is placed relative to other elements. is that which exceeds all others in weight. It has nothing of the nature of lightness. It is naturally suited to settle beneath everything else at the center, and it is light in no location. Relatively heavy grave in respectu: a body that acts as "heavy" and sinks in some environments, but acts as "light" and rises in others. is that which is not naturally suited for the center, but is light in some place in the world, specifically when compared to the absolutely heavy.

Similarly, the absolutely light is that which is naturally suited to be carried in a straight line above all moving things and to rest at the highest point without any intervening layer. It is not heavy in any part of the world. Relatively light is that which is not of this kind, but is heavy in some locations. These are the main points that Aristotle sets down at the beginning of On the Heavens original: "Celi et Mundi"; this refers to Aristotle's work De Caelo., and which Averroes Averroes, or Ibn Rushd, was a 12th-century Islamic philosopher whose commentaries on Aristotle were essential to medieval European scholarship. confirms in his fourth book.

Now we see that there is not only relative heaviness, which is water, but also absolute heaviness, which is earth. Therefore, there is not only relative lightness, which is air, but also absolute lightness, which we call fire. Water is heavy in its own place and in the places of air and fire, because it is naturally suited to descend if placed in those three areas. But it is light in the place of earth, because if it were in the place of earth, it would be carried upward to the hollow of the air.

Similarly, air is heavy in its own sphere, and therefore it is not the highest lightness. For this reason, there must be something lighter than air itself that moves in a straight line above it, and this is fire. Air is heavy when placed within the sphere of fire. Therefore, these four elements must move with rectilinear motion movement in a straight line, either up or down., unlike the fifth element The aether, which makes up the celestial bodies. which moves with circular motion.

Furthermore, since it is necessary to propose a highest lightness just as there is a highest heaviness, O. 6 b. there must be two mediums between 95 a 1. them. Because of the extreme difference between the two ends, a single medium would not be proportional. Such a medium would have to share the nature of both extremes equally.