 In late 1631, the Dutch mathematician Golius urged Descartes to consider the solution to the Pappus problem. Unlike the geometrical problems that occupied Descartes’ early researches, the Pappus problem is a locus problem, i.e., a problem whose solution requires constructing a curve—the “Pappus curve” according to Bos’s terminology—that includes all the points that satisfy the relationship stated in the problem. Generally speaking, the Pappus Problem begins with a given number of lines, a given number of angles, a given ratio, and a given segment, and the task is identify a curve such that all the points on the curve satisfy a specified relation to the given ratio. For instance, in the most basic two-line Pappus Problem  (figure 4), we are given two lines \((L_1, L_2)\), two angles \((\theta_1 , \theta_2)\), and a ratio \(\beta\). We designate \(d_1\) to be the oblique distance between a point \(P\) and \(L_1\) such that \(P\) creates \(\theta_1\) with \(L_1\), and we designate \(d_2\) to be the oblique distance between a point \(P\) in the plane and \(L_2\) such that \(P\) creates \(\theta_2\) with \(L_2\). The problem is to find all points \(P\) such that \(d_1 : d_2 = \beta\). In this case, all the sought after points \(P\) will lie along two straight lines, one line to the right of \(L_1\) and the other to the left of \(L_1\). (See  figure 5  for Bos’s presentation of the general problem.)