Full Idea
We have only to keep in mind that to find a tangent means to draw a line that connects two points of a curve at an infinitely small distance.
Gist of Idea
A tangent is a line connecting two points on a curve that are infinitely close together
Source
Gottfried Leibniz (works [1690]), quoted by Philip Kitcher - The Nature of Mathematical Knowledge 10.1
Book Reference
Kitcher,Philip: 'The Nature of Mathematical Knowledge' [OUP 1984], p.234
A Reaction
[The quote can be tracked through Kitcher's footnote]