## A quotation by Sylvestre Lacroix

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)