Full Idea
The infinitesimals are the ghosts of departed quantities.
Gist of Idea
Infinitesimals are ghosts of departed quantities
Source
George Berkeley (The Analyst [1734]), quoted by David Bostock - Philosophy of Mathematics 4.3
Book Reference
Bostock,David: 'Philosophy of Mathematics: An Introduction' [Wiley-Blackwell 2009], p.96
A Reaction
[A famous phrase, but as yet no context for it]