Thought experiments?

I once read somewhere, somewhen, that Galileo (or someone) disproved Aristotle's theory of force and gravitation without even moving or measuring projectiles. Now, as you can see, every detail about this recollection is beyond fuzzy.

INSCITIA:

Can anyone out there fill me in on this thought experiment? Who did it (if not Galileo)? What was its content?

COGITATIO:

I've tried to recall how the pure-thought experiment disproved Aristotle's theory (i.e., it was a reductio ad absurdum about the latter's claim that bigger objects fall faster), but it's just so hazy and I'm not so smart. All I remember clearly is that the author was illustrating how science is not primarily or even necessarily a mathematical endeavor. Good science depends fundamentally on observation and logic.

RESPONSUM, care of "c" [pending Cogitator analysis]:

www.ligo-wa.caltech.edu/...sci_method_9t12.html