Objection 2: Chemistry Doesn’t Study Properties
Famously, Fodor presented multiple realisation as a sign of the autonomy of the special sciences (Fodor 1974). Special sciences find “higher-level’ (in this case, functional) properties, such as being in pain, indispensable in predicting and explaining how things go, yet such properties cannot be identical with any group of physical properties if they are realised by different groups in different organisms. It is now quite common to respond to this argument by arguing that the autonomy of the special sciences may only be in the eye of the (scientific) beholder, masking a deeper ontological unity in the world. Perhaps the special sciences do not study properties at all, but instead functionally defined predicates, and the relationships between them. Here is one argument to that effect, adapted from John Heil (2003), (2012): to call something a ‘property’ is to treat it with full ontological seriousness. But it cannot be assumed that special-science predicates should be approached in this way merely because they are useful, or even indispensable in predicting and explaining how things go. Such an abundant view of properties could be assumed if they were just the intensions of the worldly shadows of meaningful predicates, but there are many reasons to set the bar higher than that. Heil (2012) proposes a less serious stance toward special-science categories, allowing that statements involving special-science predicates have truthmakers among the real. In this way, the physicalist can get vanishingly close to taking the special sciences (ontologically) seriously without actually doing so, even endorsing semantic realism.
The emergentist can reply as follows. One can think that special sciences study properties without committing the fallacy of adopting the abundant theory of properties (if fallacy it is). As we have seen, causal efficacy is often identified as a mark of reality. Being water confers on a body of matter the power to quench thirst or dissolve salt. A water molecule’s structure confers on it powers to interact in distinct ways with other species at the molecular scale. As we have seen, there is an argument about whether these powers are really inherited from physical properties, but when the Eleatic principle or Alexander’s dictum is applied, this is an argument about whether or not water is a distinct reality over and above its physical basis, or merely a dependent reality. In either case, it is a part of reality.
Consider optical activity: in the early nineteenth century, chemists and physicists noticed that some crystals possess the power to rotate plane-polarised light by a characteristic angle. Strikingly, these optically active crystals came in two forms, one which rotates light in one direction, the other rotating it in the opposite direction, by the same angle. The dissymmetrical behaviour was presumed to arise from some internal dissymmetry, but in the particles themselves, or in the way they combine to make up the crystal? In 1849, Louis Pasteur separated, by hand, crystals of the L- and D- forms of the salt of an optically active acid, sodium ammonium tartrate, which he had obtained from a racemic solution (an equal mixture of the two). Pasteur then dissolved the L- and D- crystals, and showed that they retained the rotatory power in solution, a power which must therefore reside in the individual particles of the tartrate. In the 1870s, Jacobus van ‘t Hoff and Joseph Achille Le Bel independently proposed that the optical activity arises when four different groups of atoms are attached to a single (chiral) carbon atom, giving rise to two possible structures which are non-superimposable mirror images of each other: incongruent counterparts, like right and left hands. A full physical explanation of how the structural asymmetry gives rise to the power had to wait until the 1930s (for details see Needham 2004). I have told this scientific story as if complex objects—chiral molecules, or molecular populations—can (irreduc- ibly) possess powers, such as to rotate plane-polarised light or conduct electricity. Our other candidate locus of emergence in chemistry—the emergence of a macroscopic body of stuff from its molecular constituents—we illustrated with water, and its power to conduct electricity.
Heil points out that the classes of things that fall under some special- science predicates do not exactly resemble each other. Thus, for instance,
biologists ‘abstract away’ from physical differences that would be blindingly salient from the point of view of physics, or chemistry, or, for that matter, molecular biology.
(Heil 2012: 195)
Yet it is not clear that all special-science predicates can be dismissed in this way. ‘L-tartaric acid’ is not a functional category, and the fact that it can be individuated by the structure of its molecules does not establish its reduc- ibility, as we saw in the case of water. Nor is it clear why only special-science predicates can be dismissed in this way. The point about abstraction seems unfair on the special sciences because all sciences, including fundamental physics, engage in abstraction, that is, partial consideration of the similarities among a class of objects, and ignoring the differences. Now, it might be said that fundamental physics abstracts away only from relational differences in the complex causal situations in which its objects participate. Perhaps so, but the emergentist will ask again why such relational differences cannot be irreducibly causally relevant.
For Heil, causal efficacy is not sufficient for being a property: properties can only be instantiated by substances, and substances must be simple. He does not rule out strong emergence, but properties require substances to instantiate them. In short, strongly emergent properties require emergent substances.11 This is not the place for a detailed examination of Heil’s interesting and heterodox ontological views, but I do find the idea that complex entities should be excluded a priori from ontological seriousness merely in virtue of their complexity a deeply unscientific one. The ways that complex chemical objects and situations are, in virtue of which they have causal powers, are as good a candidate as anything in science for being properties. Scientific metaphysics, it seems to me, has no business denying this. Metaphysics should not stray too far from science in what it will countenance, in the direction either of permissiveness or restriction.12