Full Idea
Galileo's most enduring contribution lay in mechanics, where he refuted the Aristotelian theory that heavier bodies fall faster than lighter.
Gist of Idea
Galileo refuted the Aristotelian theory that heavier objects fall faster
Source
Samir Okasha (Philosophy of Science: Very Short Intro (2nd ed) [2016], 2)
Book Reference
Okasha,Samir: 'Philosophy of Science: very short intro (2nd ed)' [OUP 2016], p.4
A Reaction
This must the first idea in the theory of mechanics, allowing mathematical treatment and accurate comparisons.