Reductio ad absurdum might sound like a spell from the Chamber of Secrets, but it actually refers to the practice of testing an argument by pushing it to its logical extremes.
Perhaps the most well known example is that favourite if parents everywhere: “if your friends all decided to jump off a cliff, would you do it too?”
But reductio ad absurdum does have more intellectual applications. in fact, as Daniel Dennett explains in this video, Galileo used reductio ad absurdum to demonstrate that light objects and heavy objects must fall at the same speed (accounting for air resistance) all the way back in the 15th Century:
Let’s suppose that heavier things do fall faster than light things. Now take a stone, (A), which is heavier than another stone, (B).
That means, if we tied B to A with a string, B should act as a drag on A when we drop it, because A will fall faster, B will fall slower, and so A tied to B (A+B) should fall slower than A by itself. But A tied to B is heavier than A by itself, so A+B should fall faster.
If lighter objects fall more slowly, A+B should fall both faster and slower than A by itself.
Honestly, if there wasn’t video evidence of a hammer and a feather falling at the same speed on the moon (where there’s no air resistance), I still wouldn’t be able to wrap my head around this…