"He gives the theory of involution of six points, but his definition of "involution" is not quite the same as the modern definition, first found in Fermat, but really introduced into geometry by Chasles. On a line take the point A as origin (souche), take also the three pairs of points B and H, C and G, D and F; then, says Desargues, if AB \cdot AH = AC \cdot AG = AD \cdot AF, the six points are in "involution." If a point falls on the origin, then its partner must be at an infinite distance from the origin. If from any point P lines be drawn through the six points, these lines cut any transversal MN in six other points, which are also in involution; that is, involution is a projective relation."
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Florian Cajori, A History of Elementary Mathematics: with Hints on Methods of Teaching (1897)
https://en.wikiquote.org/wiki/Girard_Desargues
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Girard Desargues
(21 February 1591 – September 1661) was a French mathematician, architect and engineer, who is considered one of the founders of . , the , and the Desargues crater are named in his honour.
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