 Note that, whereas—assuming the truth of  (ID-NEX)—two  distinct entities cannot be identity-dependent upon each other, it very plausibly is possible for each of two distinct entities to dependE for its existence on the other. For example, consider a solid sphere. Let us assume for the purposes of this example that the solid sphere is a substance, a kind of basic, ontologically independent entity (see  §6.3  for further discussion). If we think of the top and bottom “halves” of a solid sphere as being geometrically defined entities whose boundaries are specified by reference to the whole sphere of which they are equal subdivisions, then it could be argued that it is part of the essence of each such hemisphere that it exists only if the other does. (These “halves”, it should be emphasized, must not be confused with the portions of matter “filling” them at any given time, and hence should not be thought of as “parts” of the sphere of which it is materially composed.) At the same time, each hemisphere depends for its identity upon the whole sphere—one being identified as the top half of that sphere and the other as the bottom half—whereas the sphere itself does not likewise depend for its identity upon either of these halves, given that we have assumed that it is a substance in its own right. This example is admittedly controversial: one could insist that the constitutive essence for each hemisphere involves the whole sphere, but not the other hemisphere. Moreover, one could also focus on the plural or collective essence of the halves, which might help us get around this circularity (on plural/collective essence, see Zylstra 2019). Note also that, very plausibly, if x dependsE for its existence upon y, then x also dependsR for its existence upon y: essential existential dependence entails rigid existential dependence—but not, of course, vice versa.