 Abelard’s notion of the essentia of a thing is informed by his appreciation of the notion of mereological overlap (see the section on Basic Principles/Other Mereological Concepts, in the entry on  mereology).  His appreciation of mereological overlap in and of itself is a watershed, since he is one of the first (and perhaps one of the few) medieval philosophers to clearly understand this  phenomenon.[6]  Abelard’s appreciation of the notion of overlap is in evidence in his account of numerical sameness and difference (Theologia Christiana III, § 139 and §148 (1969, 247 and 250–1); for discussion see Brower 2004, 226–34). According to Abelard, x is the same in essentia as y if and only if every part of x is a part of y and every part of y is a part of x. In other words, x is the same in essentia as y if and only if x and y mereologically coincide. If x is the same in essentia as y, then x is numerically the same as y. However, if x is not the same in essentia as y, it does not follow that x is numerically distinct from y. This is because x and y could overlap—that is, share at least one part—even if they do not coincide. 