In ordinary geometry a surface is conceived as a locus of points; in Lie's geometry it appears as the totality of all the spheres having contact with the surface.

— Felix Klein, Lectures on Mathematics (1894)

Source: https://books.google.com/books?id=O2o5AQAAMAAJ&pg=PA15

From his 1893 Northwestern University lectures, published 1894.

Topics: mathematics