Full Idea
In the proposition that in all communication of motion effect and counter-effect must always be equal, not only the necessity, and thus its a priori origin, but also that it is a synthetic proposition is clear.
Gist of Idea
That force and counter-force are equal is necessary, and a priori synthetic
Source
Immanuel Kant (Critique of Pure Reason [1781], B017)
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.145
A Reaction
No, I don't follow that. God might have made Newton's Third Law that every time you push a wall it pushes you back with double force. Looks like a Humean a posteriori observation of regularity to me.