Vilfredo Pareto, referring to the equations that determine equilibrium, wrote:
These equations do not seem new to me, they are old friends. They are the equations of rational mechanics.
This quotation appears in the paper On the Economic Phenomenon: A Reply to Benedetto Croce he doesn't make clear what specific equations he's referring to. As someone with a physics background I am curious if anyone knows/could guess what he is referring to.