Full Idea
I consider in this treatise not the species of forces and their physical qualities, but their quantities and mathematical proportions.
Gist of Idea
I am studying the quantities and mathematics of forces, not their species or qualities
Source
Isaac Newton (Principia Mathematica [1687], 1.1.11 Sch)
Book Reference
Newton,Isaac: 'Philosophical Writings' [CUP 2004], p.86
A Reaction
Note that Newton is not denying that one might contemplate the species and qualities of forces, as I think Leibniz tried to do, thought he didn't cast any detailed light on them. It is the gap between science and metaphysics.