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)