25. Compare with d’Alembert’s “Introduction au Traité de dynamique ” (1743), Oeuvres, I, 393–94:

. . . in order to have a clear idea of movement we cannot avoid the necessity of differentiating, at least mentally, two sorts of extension: one which is regarded as impenetrable and which constitutes what is properly called body; the other which, being viewed simply as extension, without examining whether it is penetrable or not, is the measure of the distance from one body to another, and can serve to determine the movement or lack of movement of body.

Newton led the way in this philosophical discussion of the objects of natural science and mathematics, and Locke made several of these same comments about our ideas in his Essay Concerning Human Understanding, bk. II, chs. IV and XIII (Fraser’s edition, I, 151 ff. and 218 ff.).

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