## Sylvestre Lacroix (1765 - 1843)

The word *linear* [applied to differential equations] is improper; it is relative to Geometry, and in applying it to equations, one has in view a straight line in the equation of which the ordinate [= *y* coordinate] (or equally the abscissa [= *x* coordinate]) are found only to the first degree: thus one would not know how to regard linear equations such as *dy + Py dx = Q dx*, which belong most often to transcendental curves.

*Calculus* (1799)