What is Right with the Miracle Argument: Establishing a Taxonomy of Natural Kinds Martin Carrier* IT CERTAINLY strikes us as one of the most remarkable types of scientific achievement when apparently disparate phenomena are unified theoretically. What appeared to be disparate to the untutored eye turns out to arise from the same underlying mechanism and thus to be identical in kind. When a door slams because the windows are open and it is windy outside, this happens due to the same cause and according to the same mechanism that makes a plane lift off the ground. The prima facie conclusion is that science succeeds in going beyond the specious distinctions of the senses. It teaches us what things are truly alike. My aim in this paper is to examine the viability of this popular view. And the result will be that the view is basically correct. More precisely, I will try to show, first, that in some distinguished cases science arguably manages to induce the right classification or taxonomy among the phenomena, and that, second, this is the only access to reality that science is justifiably able to gain. Accordingly, what I am aiming to do is to support a particular and compara- tively weak form of scientific realism. Scientific realism contends that claims about certain unobservable ‘items’ which emerge from the theoretical or experimental activity of scientists are literally true; these claims faithfully refer to what is, as it were, going on behind the scenes. There is some quarrel, however, about what these ‘items’ are legitimately supposed to be. The leading brand of this doctrine is theory- realism. According to this position, the successful theories of mature science are approximately true. That is, these theories correctly portray the not- directly-observable processes and mechanisms that make the phenomena occur the way they do. A more attenuated version of scientific realism is entity-realism. On the one hand, entity-realism is an immediate consequence of theory-realism. The truth of a theory implies the existence of its theoretical entities. On the other hand, there is also a more autonomous type of entity-realism which is advanced on the basis of experiment-centered arguments. In this version, entity-realism says that the capacity to manipulate certain unobservable entities, and, in parti- cular, to manipulate them in order to experiment on something else, gives *Center for Philosophy of Science, University of Konstanz, 78434 Konstanz, Germany. Received 29 October 1992; in revised form 10 February 1993. Stud. His. Phil. Sci., Vol. 24, No. 3, pp. 391-409, 1993. 0039-368 l/93 S6.00 + 0.00 Printed in Great Britain 0 1993. Pergamon Press Ltd 391 392 Studies in History and Philosophy of Science strong evidence for the real existence of these entities. Entity-realism is thus non-committal as to the (near) truth of the accepted scientific account of these entities. It is no theory’s entity but the experimenter’s entity to which actual existence is attributed. Belief in these entities does not imply belief in any one of the theories involved. An even more attenuated variant of realism, and the one I set out to support, is a realism of kinds. The claim is that science, at least on some rare and distinguished occasions, manages truthfully to forge links among the phenomena. Science sometimes succeeds in collecting phenomena into equiva- lence classes that reflect truly existing similarity relations among these pheno- mena. Whereas entity-realism implies a kind-realism, the reverse does not hold. It will be seen that commitment to the experimentally established entities entails commitment to the kind-structures they introduce. By contrast, the assumption that science is sometimes able to unveil true relations of similarity does not imply that the mechanisms employed to establish these relations and the entities they invoke reflect anything in nature. The argument comes in three steps. The first one is introductory. I sketch the notion of ‘natural kinds’ and show, using some examples from the history of science, that the structure of natural kinds is theory-dependent and indeed changes in the course of scientific progress. This finding implies that a naive form of a realism of kinds is untenable empirically. In the second step I consider the experimental route to natural kinds. The prima facie advantage of this alternative is that it seems to be exempt from the uncertainties and vicissitudes of theoretical reasoning. It turns out, however, that this position is likewise beset with a historical counter-example. Next comes, finally, the constructive step. I take up some earlier results of mine about the actual impact of the so-called ‘Miracle Argument’ in favor of scientific realism. The argument says that scientific realism is a necessary precondition for any explanation of scientific progress; it is the only choice for divesting progress of its miraculous aspects. I will argue that the Miracle Argument is insufficient for achieving its proper objective, namely, theory-realism, and that its real accom- plishment consists in showing that there are instances in which science provides us with a veridical portrait of relations of similarity as they prevail in nature. 1. Changing Patterns of Natural Kinds Every theory determines a classification among the objects or events it deals with; it induces a taxonomy into its universe of discourse. The laws of the theory are framed using certain descriptive predicates, and the relevant objects obey the laws in virtue of satisfying these predicates. A law thus serves to bind several objects together. It establishes a link of similarity among them by What is Right with the Miracle Argument 393 regarding them as instances of the same law. A so-created equivalence class of like objects is called a natural kind (cf. Fodor 1974, 101-102). Take, for example, the law: All electrons possess the elementary charge e. This law generates the natural kind ‘electron’ by quantifying over electrons. Electrons constitute a natural kind because there is a law that applies to them in virtue of their property of being electrons. Different laws may well pick the same natural kind. All further laws about electrons (such as: All electrons have a spin-value of )) obviously specify the same kind, namely, the class of electrons. This implies immediately that a theoretical change need not involve a change in the concomitant kind-structure. Different bodies of laws may select the same objects as being alike.’ The concept of natural kinds, as just circumscribed, is entirely non- committal as to the issue of scientific realism. Natural kinds are created by a corpus of laws irrespective of whether these laws are interpreted merely as useful but fictitious unifiers or are thought to refer to a theory-independent reality. Accordingly, natural kinds need not be truly natural, and the attribute ‘natural’ is, strictly speaking, a misnomer. In order to avoid the realist overtones the expression ‘scientific kinds’ has been proposed, but this proposal seems not to have gained acceptance. Thus I stick to the usual term, taking its non-committal sense to be understood. The reality of natural kinds constitutes the topic of the present investigation. The first response to this problem presumably is that we are surely entitled to interpret the taxonomy specified by present-day science realistically. After all, its laws have undergone a large number of severe tests; they are well- confirmed and thus deserve our confidence. And this confidence quite naturally extends to the taxonomy induced by these laws. Most of us would thus be inclined to reason as follows. Since present-day knowledge includes laws about electrons, electrons form a natural kind. For this reason, electrons are in reality of the same kind. Let’s examine the tenability of this assessment. A condition familiar from the discussion of scientific realism in general is the following retention requirement. For a theory component to be interpreted realistically, it is necessary that it be retained across scientific change. In particular, if earlier successful theories in the mature sciences are approxi- mately true, then the later even more successful theories in the same discipline ‘Note that ‘natural kind’ is used here as a technical term which does not wholly coincide with everyday usage. Biological species, for instance, though they are natural kinds in a rough and ready sense, hardly qualify as natural kinds in the present understanding. Kinds are derived from laws, and it is a matter of dispute in current philosophy of biology whether there are any specifically biological laws. In any event, a regularity of the sort ‘All ravens are black’ certainly does not count as a law of nature and thus does not give rise to the natural kind ‘raven’. 394 Studies in History and Philosophy of Science will preserve the earlier laws as limiting cases (cf. Putnam 1978, 2&24; Laudan 1981, 234240). Applied to natural kinds this means that the kind-structure of earlier well-confirmed theories should be reproduced ‘in the limit’ by the kind- structure of their respective successor theories. The intuition behind the retention condition is as follows. Once-successful but now-rejected theories in the mature sciences were in their lifetimes well- confirmed according to our present methodological criteria. If such theories turn out to be wrong on all counts, nothing prevents us from the meta- inductive inference that our present most cherished accounts are likewise doomed to complete failure and that nothing will remain from them in the end. If scientific realism is supposed to be a viable position, this meta-inductive move has to be blocked; and it can only be blocked by assuming that these earlier accounts indeed got something right (cf.