Physics VIII.4-5 Exegesis

In lines 255a24-255b31 of Aristotle’s Physics, Aristotle tries to explain the “motion of heavy and light things to their places”. Aristotle does this by explaining in terms of potentiality what an elements potential movement is. Aristotle notes, “Whenever something capable of acting and something capable of being acted on are together, what is potential becomes actual.” By this he seems to mean that when thing A, which is potentially thing B, becomes B, it becomes actually B, and potentially C. Aristotle uses this distinction to talk about the elements’ natural movements. Aristotle needs a further mechanism, though, to explain why the elements are not consistently changing. He does this by suggesting that something prevents or hinders a change. For example, Aristotle thinks that cold is potentially hot, but has something hindering its movement so that all things do not progressively get hotter. Aristotle seems to think that if you remove the thing that is hindering cold, it will go from potentially hot to actually hot. Aristotle also uses the example of potentially light things becoming light.

Aristotle also thinks that an element can change into another element and still have some other potential change. For example, Aristotle notes that water is potentially air, which is potentially light. When water becomes lighter, it goes from potentially being air to actually being air. As air though, the once-water still has the potential to become lighter, and thus rise further. All that is needed is for some hindrance to be removed. This is much like a hand cupping some air under water; the air has the potential to move upwards, but can’t since a hand is hindering it. When the hand is moved, the air actualizes its potentiality and moves upward until it meets with another hindrance.

Finally, Aristotle claims that in all cases, the element is not moving itself, but “...contains within itself the source of motion – not of moving something or of causing motion, but suffering it.” Aristotle’s motivation for making this claim is to show that the elements do not move naturally like animals and humans, but accidentally, through other causes. Aristotle explains how the elements movements are accidental by looking at some parallel examples and seeing how people talk about them. Aristotle notes that when a person removes an obstacle to a pillar’s fall, properly speaking, we say that the person caused the pillar to fall, not that the pillar moved itself. In the same way, when an element changes form, it is because something that hinders its change has been removed.

It is a bit unclear whether Aristotle thinks the removal of hindrances needs to be done by natural things. If lightning hit a tree, and removed the hindrances from being hot and dry, I’m not sure if Aristotle would say that the lightning caused the tree to burn, or if he would maintain that some natural being removed an obstacle from an element, through which in a chain of events caused the tree to burn. (i.e. a butterfly flaps its wings, removes some small hindrance in the air, which leads to the removal of other hindrances, snowballing into the lightning being formed.)