Full Idea
Descartes incorporated the conservation of what he called 'quantity of motion', by which he meant mass times speed.
Gist of Idea
Descartes said there was conservation of 'quantity of motion'
Source
report of René Descartes (The World [1631]) by David Papineau - Thinking about Consciousness App 2
A Reaction
Mass times velocity is now called 'momentum'. Is this the first ever conservation law? There are now lots of them.
Book Reference
Papineau,David: 'Thinking about Consciousness' [OUP 2004], p.234