 In addition to the modal characterization and the definitional characterization, there is yet another way of characterizing the notion of an essential property. It agrees with the definitional characterization that the modal characterization is too liberal in what it counts as essential, but it avoids appeal to the notion of a definition of an object. On the explanatory characterization, the essential properties of an object are the object’s deepest explanatory properties—those properties that figure fundamentally into explanations of the object’s possessing the other properties it does. (For example, having six protons might count as an essential property of a carbon atom because this property figures fundamentally into explanations of its possession of other properties, like its bonding characteristics.) This sort of account threatens to make the essential/accidental property distinction relative, since what counts as explanatorily primary seems to depend on the interests and abilities of the explainers. Some advocates of the explanatory characterization, such as Meghan Sullivan (2017), are willing to accept that the distinction is relative in some sense. Others, such as Irving Copi (1954) and Michael Gorman (2005), hold that there is a more “metaphysical” and less “epistemic” understanding of the notion of explanation, according to which one thing explains another thing just in case there is a certain mind-independent relation that holds between them. Just as the advocate of the definitional characterization is challenged to provide a respectable understanding of the relevant notion of definition, the advocate of the explanatory characterization is challenged to provide (or to borrow from the philosophy of science) a respectable understanding of the relevant notion of explanation.