 The principal objects of intuition are “simple natures”. Simple natures are not propositions, but rather notions that are “so clearly and distinctly [known] that they cannot be divided by the mind into others which are more distinctly known” (AT 10: 418, CSM 1: 44). The simple natures are, as it were, the atoms of human knowledge (Hamelin 1921: 86); all other notions and propositions are composed of simple natures. Descartes divides the simple natures into three classes: intellectual (e.g., knowledge, doubt, ignorance, volition, etc.), material (e.g., extension, shape, motion, etc.), and common (e.g., existence, unity, duration, as well as common notions “whose self-evidence is the basis for all the rational inferences we make”, such as “Things that are the same as a third thing are the same as each other”, etc., AT 10: 419, CSM 1: 45). The intellectual simple natures must be intuited by means of the intellect alone. The material simple natures must be intuited by means of the intellect aided by the imagination. The common simple natures may be intuited either by the intellect alone or the intellect aided by the imagination (ibid.). The intellectual simple natures define the essence of mind (one of the objects of Descartes’ metaphysics) and the material simple natures define the essence of body (the object of Descartes’ mathematics and natural philosophy).