Full Idea
The method of infinitesimals is that you divide by the time, and then set the time to zero.
Gist of Idea
With infinitesimals, you divide by the time, then set the time to zero
Source
Philip Kitcher (The Nature of Mathematical Knowledge [1984], 10.2)
Book Reference
Kitcher,Philip: 'The Nature of Mathematical Knowledge' [OUP 1984], p.238