 He goes on to say that “Unity is the common nature which, we said above, all things which we are comparing must participate in equally.” (AT X 449; CSM I 63 See also AT X 440f; CSM I 57f) The simple natures shape and size share in common their presupposing the simple nature extension. Extension is the common nature; it unites such natures into a single thing (a body). Thought or thinking is the common nature that unites the other simple natures into a single thing (a mind). An idea is said to be obscure, then, whenever no necessary conjunction (the “presupposes” relation) is exhibited or made explicit in an idea.