 There is a further question surrounding Descartes’ criterion for “geometric” curves. As we have seen above, Descartes’ explicit concern in Book Two is to offer a standard for geometrical curves that is bound with intelligible, clear and distinct motions needed for their construction. However, Mancosu (2007) offers a compelling case that behind Descartes’ explicit remarks in La Géométrie lies a more fundamental concern: To ensure that those curves mathematicians had used to square the circle, such as the spiral and quadratrix which are explicitly mentioned in Book Two, are rendered non-geometrical. Mancosu supports his case with evidence from Descartes’ correspondence that shows, for Descartes, it is in fact possible in some instances to clearly and distinctly conceive the relation between a straight line and a circle, a relation he had considered inexact in La Géométrie. Namely, in a 1638 letter to Mersenne, Descartes writes,