Full Idea
When we have gone so far as to deprive the length of its breadth altogether, we no longer conceive even the length, but along with the removal of the breadth the conception of the length is also removed.
Gist of Idea
If we try to conceive of a line with no breadth, it ceases to exist, and so has no length
Source
Sextus Empiricus (Against the Physicists (two books) [c.180], I.392)
Book Reference
Sextus Empiricus: 'Against the Physicists/Against the Ethicists', ed/tr. Bury,R.G. [Harvard Loeb 1997], p.187
A Reaction
The only explanation of our retaining an understanding of a line even after we have removed its breadth is that we have abandoned experience and conceptualised the line - by idealising it.