 In addition to artifacts, plants, and animals, some medieval philosophers expand the class of continuous integral wholes to cover homogeneous masses, such as some gold or some water, and the class of disaggregated integral wholes to include scattered mereological sums, such as the sum of this mountain and this dog. However, one will not find any medieval philosopher who assents to the modern mereological thesis of universalism, or what David Lewis calls “unrestricted fusion” (1991, 74; cf. Simons 1987, 108–12 and the subsection on unrestricted fusions in the Encyclopedia entry on  mereology).  Peter Abelard comes close when he asserts that any two items, even ones belonging to different categories of being, can constitute a plurality (Dialectica 548.19–22; see also the anonymous twelfth-century Introductiones Montanae maiores, 69rb, where Abelard’s thesis is mocked). But one suspects that even Abelard, if pressed, might step back from assenting to a full-blown version of universalism for theological reasons. For in discussions of God’s simplicity, it is routinely noted that God is not intrinsically composite and that He cannot be combined with anything else and that nothing can be combined with Him (see, e.g., Aquinas Summa Theologiae I, q. 3, arts. 7–8). This strong thesis about the lack of composition in God is maintained because being a part of something implies a certain sort of potentiality and incompleteness. If anything could be a part of a whole that also has God as a part, then there is something that God could be–namely, a part of this composite–that He currently is not. And even if He currently is a part of the sum of all things (for instance), given that some of the other parts are contingent beings, the sum of all things is contingent. Accordingly, God’s parthood is contingent, or has a potentiality. But God is fully actual and necessary. Hence, God cannot be a part of some greater  composite.[3] 