20. And first, I have so laid out the plan of the whole work as to consult the advantage of the reader by the logical order in which its books are arranged. It has been my resolve to publish no half-finished and ill-considered treatise, lest its disorderly array should resemble the confused clamour of a mob of peasants. And since no one can scale a precipice unless there be jutting ledges to aid his progress to the summit, I have here set down in order the primary outlines of our ascent leading our difficult course of argument up the easiest path; not cutting steps in the face of the rock, but levelling it to a gentle slope, that so the traveller, almost without a sense of effort may reach the heights.