BOOK I PART II
the one can never afford us a perfect standard
for the other. An exact idea can never be built
on such as are loose and undetermined.
The idea of a plain surface is as little suscep-
tible of a precise standard as that of a right line;
nor have we any other means of distinguish-
ing such a surface, than its general appearance.
It is in vain, that mathematicians represent a
plain surface as produced by the flowing of a
right line. It will immediately be objected, that
our idea of a surface is as independent of this
method of forming a surface, as our idea of an
ellipse is of that of a cone; that the idea of a
right line is no more precise than that of a plain
surface; that a right line may flow irregularly,
and by that means form a figure quite different
from a plane; and that therefore we must sup-
pose it to flow along two right lines, parallel to