Carving Nature at Its Joints: Natural Kinds in Metaphysics and Science

3. It Takes More Than All Kinds to Make a World

3 1 Introduction Figure 3.1 shows one picture of what it takes to make a world: the kinds of particles, the kinds of forces, and a vast mixing bowl. I shall be arguing that although, as the saying goes, “it takes all kinds to make a world,” there is an important sense in which it takes more than all of the actual kinds. What the particle species would have been like, had the list of species been different in various ways, is crucial to making the actual classes the natural kinds they are. The various species of elementary particle (some of which are depicted below, in table 3.1) are in many ways ideal cases of natural kinds. Each elementary particle belongs to exactly one of these natural kinds and it essentially belongs to that kind. In terms of certain properties, there are perfect uniformities within each species and sharp distinctions between the species: there are no intermediate cases, much less a continuum of intergradations. Among these characteristic properties, some are fundamental and suffice to give necessary and sufficient conditions for species-membership, while others derive from the fundamental properties via exceptionless uniformities with no ceteris paribus escape clauses. The particles belonging to these species are elementary, so there are no worries that these classes are actually heterogeneous at a more basic level. All in all, the elementary-particle species are ideally behaved natural kinds. (Of course, perhaps charge, charm, and the other properties I have identified in the table as fundamental are not actually fundamental; perhaps the particles I have listed are not even elementary. I will assume they are, and I will assume various other physical details along the way— but merely for the sake of having definite examples. Nothing I have to say will turn on these assumptions.) It Takes More Than All Kinds to Make a World Marc Lange 54 M. Lange Table 3.1 Some elementary particles of matter. Fundamental properties Some derivative properties quark spin charge* baryon number S C B T mass (MeV/c2 ) magnetic moment (µN) half-life decay products (with W) Up 1/2 2/3 1/3 0 0 0 0 360 +1.852 >4.6 × 1012 yr d Down 1/2 –1/3 1/3 0 0 0 0 360 –0.972 >900 s u Charm 1/2 2/3 1/3 0 1 0 0 1500 ? 1.1 × 10–12 s s(95%), d(5%) Strange 1/2 –1/3 1/3 –1 0 0 0 540 –0.613 1.24 × 10–8 s u Top 1/2 2/3 1/3 0 0 0 1 174000 ? 10–25 s b(>99%), s, d Bottom 1/2 –1/3 1/3 0 0 –1 0 5000 ? 1.3 × 10–12 s c(>99%), d, u lepton spin charge* lepton number mass (MeV/c2 ) magnetic moment (10–27 J/T) half-life decay products Electron 1/2 –1 1 0.510998918 –9284.764 stable Muon 1/2 –1 1 105.658369 –44.90447 2.2 × 10–6 s e– & υe& υμ Taon 1/2 –1 1 1776.99 –2.6801 2.9 × 10–13 s e– & υt & υe (17.84%), μ– & υt & υμ (17.36%) . . . *1 unit = 1.602 176 487(40) × 10–19 C. It Takes More Than All Kinds 55 Beyond the sorts of facts depicted in table 3.1, at least three further ingredients are needed to make these classes into natural kinds. First, the fundamental properties shared within a class and collectively differentiating the classes must be natural properties: not gerrymandered, “wildly disjunctive” (Fodor 1974, 103), “grue”-some (Goodman 1983) shadows of predicates. Members of the same class must be genuinely alike and members of different classes genuinely dissimilar in various respects.1 Second, the combination of fundamental properties characteristic of the muon, for example, must not merely be universally associated with each of the muon’s derivative properties. Rather, they must be connected by natural law. It must be no accident that any two bodies with the muon’s charge, mass, and so forth also possess the muon’s half-life and magnetic moment. Third, going beyond the muon or any other particular line of the table, it must be no accident that elementary particles alike in every one of the fundamental respects are alike in the derivative respects. Of course, that result can be achieved in a cheap way: namely, for the...