The old rule was part of Aristotle’s larger theory of natural motion. Earthy and watery bodies were thought to move naturally toward the center of the cosmos, while air and fire tended upward. In his Physics, Aristotle reasoned that, other things being equal, a body with greater weight should move faster through a resisting medium. That made falling speed a consequence of both the body’s heaviness and the density of what it moved through.
This was not an isolated guess that scholars repeated unchanged for two thousand years. Aristotle’s physics became a framework to argue with. Medieval natural philosophers criticized, modified, and extended parts of it; discussions of projectile motion and impetus are good examples. But when Aristotle’s translated natural works entered the medieval universities, his Physics became one of the central texts through which motion itself was studied.
The decisive early-modern change was therefore methodological as well as numerical. Galileo idealized falling motion, used inclined planes and mathematical relationships, and separated the behavior of falling bodies from the everyday effects of air resistance. In a vacuum, objects with different masses share the same gravitational acceleration; differences seen in ordinary air usually come from drag, shape, and buoyancy.
The longevity of this card is a reminder that an old theory can remain intellectually productive even while parts of it are being challenged. Medieval criticism of Aristotle helped create some of the conceptual tools later used to move beyond him.