Full Idea
Law 2: A change in motion is proportional to the motive force impressed and takes place along the straight line in which that force is impressed.
Gist of Idea
2: Change of motion is proportional to the force
Source
Isaac Newton (Principia Mathematica [1687], Axioms)
Book Reference
Newton,Isaac: 'Philosophical Writings' [CUP 2004], p.71
A Reaction
This gives the equation 'force = mass x acceleration', where the mass is the constant needed for the equation of proportion. Effectively mass is just the value of a proportion.