... Enclosed a mathematical essay by Moore. The conclusion that the algebraic method is only the differential method disguised refers of course only to his own method of geometrical construction and is pretty correct there, too. 1 have written to him that you place no value on the way the thing is represented in geometrical construction, the application to the equations of curves being quite enough. Further, the fundamental difference between your method and the old one is that you make x change to x', thus making them really vary, while the other way starts from x + h, which is always only the sum of two magnitudes, but never the variation of a magnitude. Your x therefore, even when it has passed through x and again becomes the first x, is still other than it was; while x remains fixed the whole time, if h is first added to it and then taken away again. However, every graphical representation of the variation is necessarily the representation of the completed process, of the result, hence of a quantity which became constant, the line x; its supplement is represented as x + h, two pieces of a line. From this it already follows that a graphical representation of how x', and then again becomes x, is impossible ...
Letters: Marx-Engels Correspondence 1882