Full Idea
A force is the supposed cause of acceleration, ...but an acceleration is a mere mathematical fiction, a number, not a physical fact.
Gist of Idea
Force is supposed to cause acceleration, but acceleration is a mathematical fiction
Source
Bertrand Russell (The Principles of Mathematics [1903], §448)
Book Reference
Russell,Bertrand: 'Principles of Mathematics' [Routledge 1992], p.474
A Reaction
This rests on his at-at theory of motion, in Idea 14168. I'm not sure that if I fell off a cliff I could be reassured on the way down that my acceleration was just a mathematical fiction.
Related Idea
Idea 14168 Occupying a place and change are prior to motion, so motion is just occupying places at continuous times [Russell]