Full Idea
We can abstract the direction of a line by taking the direction as the equivalence class of that line and all lines parallel to it. There is no subtraction of detail, but a multiplication of it; by swamping it, the specifics of the original line get lost.
Gist of Idea
The abstract direction of a line is the equivalence class of it and all lines parallel to it
Source
David Lewis (On the Plurality of Worlds [1986], 1.7)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.85
A Reaction
You can ask how wide a line is, but not how wide a direction is, so a detail IS being subtracted. I don't see how you can define the concept of a banana by just saying it is 'every object which is equivalent to a banana'. 'Parallel' is an abstraction.