 As for option (ii)—to the effect that de re mereological indeterminacy is a matter of degree—the picture is different. Here the main motivation is that whether or not something is part of something else is really not an all-or-nothing affair. If Tibbles has two whiskers that are coming loose, then we may want to say that neither is a definite part of Tibbles. But if one whisker is looser than the other, then it would seem plausible to say that the first is part of Tibbles to a lesser degree than the second, and one may want the postulates of mereology to be sensitive to such distinctions. This is, for example, van Inwagen's (1990) view of the matter, which results in a fuzzification of parthood that parallels in many ways to the fuzzification of membership in Zadeh's (1965) set theory, and it is this sort of intuition that also led to the development of such formal theories as Polkowsky and Skowron's (1994) “rough mereology” or N. Smith's (2005) theory of “concrete parts”. Again, there is room for some leeway concerning matters of detail, but in this case the main features of the approach are fairly clear and uniform across the literature. For let π be the characteristic function associated with the parthood relation denoted by the basic mereological primitive, ‘P’. Then, if classically this function is bivalent, which can be expressed by saying that π(x, y) always takes, say, the value 1 or the value 0 according to whether or not x is part of y, to say that parthood may be indeterminate is to say that π need not be fully bivalent. And whereas option (i) simply takes this to mean that π may sometimes be undefined, option (ii) can be characterized by saying that the range of π may include values intermediate between 0 and 1, i.e., effectively, values from the closed real interval [0, 1]. In other words, on this latter approach π is still a perfectly standard, total function, and the only serious question that needs to be addressed is the genuinely mereological question of what conditions should be assumed to characterize its behavior—a question not different from the one that we have considered for the bivalent case throughout the preceding sections. 