  The distinction between simple substances and aggregates becomes an important one in Leibniz's philosophy. To Arnauld, he writes the following: “I hold that philosophy cannot be better reestablished and reduced to something precise, than by recognizing only substances or complete beings endowed with a true unity, together with the different states that succeed one another; everything else is only phenomena, abstractions, or relations” (G II 101/AG 89). If this is the case, then aggregates of simple substances are merely phenomena and fail to have the reality of the underlying simples. Further, the bodies of natural philosophy, the bodies of the world we observe around us, would seem to be in some sense mere phenomena. 