 Given these remarks, the fundamental problem with the spiral, the quadratrix, and “lines that are like strings” is that their construction requires consideration of the ratio, or relation, between a circle and straight line. Consider the spiral. As we saw above, its construction involves two uniform motions, namely, the uniform rectilinear motion of a point along a segment and the uniform circular motion of the segment around a point. These two motions must simultaneously be considered in order for the moving point’s path to describe the spiral, and this, for Descartes, is what is ultimately problematic. The human mind can think about simultaneous rectilinear and circular motions, but it cannot do so with the clarity and distinctness required to meet the exact and rigorous standards of geometry. (This claim is not without its problems, which will be discussed in  section 3.3  below. For a comparison between Descartes’ criterion for the construction of geometrical curves and the views put forward by Pascal, see Jesseph 2007.)