 You ask me if I think that a sphere which rotates on a plane describes a line equal to its circumference, to which I simply reply yes, according to one of the maxims I have written down, that is that whatever we conceive clearly and distinctly is true. For I conceive quite well that the same line can be sometimes straight and sometimes curved, like a string (To Mersenne, 27 May 1638; AT 2, 140–141; translation from Mancosu 2007, 118).