 Concerning reflexivity (16), two sorts of worry may be distinguished. The first is that many legitimate senses of ‘part’ just fly in the face of saying that a whole is part of itself. For instance, Rescher (1955) famously objected to Leonard and Goodman's theory on these grounds, citing the biologists' use of ‘part’ for the functional subunits of an organism as a case in point: no organism is a functional subunit of itself. This is a legitimate worry, but it appears to be of little import. Taking reflexivity (and antisymmetry) as constitutive of the meaning of ‘part’ simply amounts to regarding identity as a limit (improper) case of parthood. A stronger relation, whereby nothing counts as part of itself, can obviously be defined in terms of the weaker one, hence there is no loss of generality (see Section 2.2 below). Vice versa, one could frame a mereological theory by taking proper parthood as a primitive instead. As already Lejewski (1957) noted, this is merely a question of choosing a suitable primitive, so nothing substantive follows from it. (Of course, if one thinks that there are or might be objects that are not self-identical, for instance because of the loss of individuality in the quantum realm, or for whatever other reasons, then such objects would not be part of themselves either, yielding genuine counterexamples to (16). Here, however, we stick to a notion of identity that obeys traditional wisdom, which is to say a notion whereby identity is an equivalence relation subject to Leibniz's law.) The second sort of worry is more serious, for it constitutes a genuine challenge to the idea that (16) expresses a principle that is somehow constitutive of the meaning of ‘part’, as opposed to a substantive metaphysical thesis about parthood. Following Kearns (2011), consider for instance a scenario in which an enduring wall, W, is shrunk down to the size of a brick and eventually brought back in time so as to be used to build (along with other bricks) the original W. Or suppose wall W is bilocated to my left and my right, and I shrink it to the size of a brick on the left and then use it to replace a brick from W on the right. In such cases, one might think that W is part of itself in a sense in which ordinary walls are not, hence that either parthood is not reflexive or proper parthood is not irreflexive. For another example (also by Kearns), if shapes are construed as abstract universals, then self-similar shapes such as fractals may very well be said to contain themselves as parts in a sense in which other shapes do not. Whether such scenarios are indeed possible is by itself a controversial issue, as it depends on a number of background metaphysical questions concerning persistence through time, location in space, and the nature of shapes. But precisely insofar as the scenarios are not obviously impossible, the generality and metaphysical neutrality of (16) may be questioned. (Note that those scenarios also provide reasons to question the generality of many other claims that underlie the way we ordinarily talk, such as the claim that nothing can be larger than itself, or next to itself, or qualitatively different from itself. Such claims might be even more entrenched in common sense than the claim that proper parthood is irreflexive, and parthood reflexive; yet this is hardly a reason to hang on to them at every cost. It simply shows that our ordinary talk does not take into account situations that are—admittedly—extraordinary.)