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In the mathematics I can report no deficience, except that it be that men do not sufficiently understand the excellent use of the pure mathematics, in that they do remedy and cure many defects in the wit and faculties intellectual. For if the wit be too dull, they sharpen it; if too wandering, they fix it; if too inherent in the sense, they abstract it. So that as tennis is a game of no use in itself, but of great use in respect it maketh a quick eye and a body ready to put itself into all postures; so in the mathematics, that use which is collateral and intervenient is no less worthy than that which is principal and intended.
John Fauvel and Jeremy Gray (eds.) A History of Mathematics: A Reader, Sheridan House, 1987.
Rene Descartes' Treatise on Method
This is the title page to René Descartes' Treatise on Method, with its three famous essays on Dioptrics, Meteorology, and Geometry. The third essay contains Descartes' treatment of analytic geometry as well as his theory of equations and his rule for finding the normal to a given curve.
A complete translation of the Treatise with the essays is in René Descartes, Discourse on Method, Optics, Geometry, and Meteorology, trans by Paul Olscamp (Indianapolis, Bobbs-Merill Co., 1961).