 He divides the Rules into three principal parts: Rules 1–12 deal with the definition of science, the principal operations of the method (intuition, deduction, and enumeration), and what Descartes terms “simple propositions”, which “occur to us spontaneously” and which are objects of certain and evident cognition or intuition (e.g., “a triangle is bounded by just three lines”) (see AT 10: 428, CSM 1: 50; AT 10: 368, CSM 1: 14). Rules 13–24 deal with what Descartes terms “perfectly understood problems”, or problems in which all of the conditions relevant to the solution of the problem are known, and which arise principally in arithmetic and geometry (see AT 10: 429–430, CSM 1: 51); Rules 25–36 deal with “imperfectly understood problems”, or problems in which one or more conditions relevant to the solution of the problem are not known, but must be found. These problems arise for the most part in natural philosophy and metaphysics. The Rules end prematurely at Rule 21 (see AT 10: 428–430, CSM 1: 50–51).