Aristotle’s idea that the speed of a body in free fall depends on its weight permeated physics for centuries, until Galileo Galilei came to revolutionize this and other bases that are now part of how we understand the forces of the universe.
Already in Ancient Greece, Aristotle developed his own conclusions about the behavior of bodies in different fluids, including air. Since then, the Greeks were already wondering how objects tend to fall to the ground and what force is behind this. For this reason, Aristotle developed his primitive gravitational theory, which among many aspects of it here stated that the free fall of objects depended on the ‘lightness’ or ‘gravity’ of the weights. That is, the heavier an object, the faster it would fall to the ground. On the contrary, the free fall of lighter objects would tend to be delayed when purchased with heavier objects.
And this idea remained present until Galileo Galilei came out to refute it in a masterful way. To prove that Aristotle was wrong, Galileo climbed (hypothetically) to the top of the Tower of Pisa and dropped two objects, one heavy and one lighter. If Aristotle was right, then the heavier object must fall first. And although Galileo did not appeal to practice, since there are no records of him actually carrying out the experiment, he did appeal to logic and by mere reductio ad absurdum he deduced that Aristotle had erred. How did he achieve it?
Reduced to the absurd
He used the redoubt of the absurd in a masterful way just by mentally carrying out the experiment described above. The same thing that is mentioned in different sources, but is almost never described in full and that ‘almost’ makes an enormous difference in understanding why Galileo concluded that Aristotle was wrong.
In contemporary wisdom it is mentioned that Galileo dropped two objects with different weights from the Tower of Pisa. What is not mentioned is that it was actually a system, not isolated objects. Thus, the Italian physicist imagined both orbs joined by a rope and therefore one would influence the result of the other and vice versa. If the Aristotelian theory were correct, we would obtain two conclusions that are contradictory, an argument that in logic is called ‘reductio ad absurdum’.
On the one hand, we would have that the lighter stone would slow down its companion (just as the heavier one would accelerate the first one). While on the other hand, the weight of both added together would constitute a greater weight than that of the objects separately. So it should fall faster.
From the same event we obtain two conclusions that are contradictory: in the first case, the whole should fall slower than if only the heavy object is thrown. In the second case, the whole should fall faster than if only the largest stone were thrown. This is how Galileo finally demonstrated that Aristotle had been wrong in his conclusions.