… What a fish is is not the sort of thing a scientist (except, perhaps a linguist) could find out.
This is from the essay ‘Are Whales Fish?’ in Humans and Other Animals by John Dupré (2002):
… the notion that there is a ‘scientific’ usage of the word ‘fish’ is a decidedly suspect one. The appeal in the definition to such technical matters as the possession of gills or cold-bloodedness seems rather a quasi-scientific rationalization of an extra-scientific linguistic intuition than the report of a genuinely scientific usage. Indeed, as is so commonly the case with attempts to define biological kinds, it is not even strictly true of all its intended referents. Some species of tuna maintain body temperatures as much as 20 degrees higher than their surroundings, and so should qualify as warm-blooded. And the lungfish Protopterus has been shown to get only 10 percent of its oxygen from water through its reduced gills. If its gills were to disappear completely at a subsequent evolutionary stage, I doubt whether it would thereby cease to be a fish. Given, then, that neither ‘whale’ nor ‘fish’ is really a scientific term, the rationale for the dictum taught religiously to all our children that whales are not fish (and it is interesting that it is something that reliably requires to be taught) is more than a little unclear.
… Despite the foregoing considerations, I have already conceded the obvious fact that whales are not fish. Why not? We might ask both why this fact is obvious, and why it came to be a fact at all. The first question is easy enough. Educated speakers will, I suspect, almost unanimously refuse to apply the word ‘fish’ to Blue whales, Killer whales, and similar creatures and for that matter, to dolphins. Ultimately I suppose that this is the only relevant evidence, and that it is decisive. On the other hand, I also suspect that if pushed to rationalize this linguistic intuition, most people will be found to believe that scientists have found out what fish are, and what whales are, and that the latter are distinct from the former. Here, as I have argued, they would be mistaken. What a fish is is not the sort of thing a scientist (except, perhaps a linguist) could find out.
Much more interesting, then, is the second question. Is there a good reason for teaching our children that whales are not fish? Even if these are not scientific categories, one might argue that some useful scientific knowledge is transmitted by using them this way. Whales are, after all, mammals, and no other mammals are much like fish. Being mammals ourselves, we tend to know quite a bit about this class or organism, and we certainly learn a good deal about whales by knowing that they are mammals. But this argument is not compelling. The obvious rejoinder is that some mammals are fish. In fact, if we taught our children that whales were mammalian fish, they would both learn to apply general knowledge about mammals to whales (they bear live young and suckle them, are warm-blooded, etc.) and might learn that ‘fish,’ unlike ‘mammal,’ was not a term for any coherent scientific grouping of organisms but a loose everyday term for (perhaps) any aquatic vertebrate. Indeed, the argument that because whales are mammals they cannot be fish seems to me to be a paradigm for the confusion between scientific and ordinary language biological kinds.
… ordinary language classifications are typically quite as well motivated, and the kinds to which they refer may be just as objectively real, as biological classifications. It is just that they are differently motivated.
… I want to claim, however, that these imported terms from scientific discourse should be understood quite differently from more familiar and well entrenched ordinary language terms, and that the failure to make this distinction indicates a significant confusion common to many philosophers and lexicographers. Whereas the definition of mammal as, say, ‘warm-blooded, hairy vertebrate with four-chambered heart and which nourishes its young with milk from maternal mammary glands’ is entirely appropriate, the definition of fish as ‘cold-blooded, aquatic vertebrate with gills, and (usually) scales, fins, etc.’ is much more questionable. These definitions look very much alike. But, I have suggested, the terms to which they apply are of quite different kinds. thus, while it is quite appropriate to say that it is a scientifically attested fact that all mammals have four-chambered hearts, it seems to me something like a category mistake to say that it is a scientific fact that all fish have gills — not because they might not but because science has nothing to say about all fish.
This last remark perhaps gets to the heart of the present problem. It is, I think, not widely accepted that there are any matters of much importance about which science has nothing to say. And certainly the question, ‘What kind does this organism belong to?’ will strike most people as paradigmatically the kind of question about which science must be the only authoritative arbiter. Ironically, perhaps, the idea of an authoritative and unique answer to such a question really assumes some version of essentialism, the idea that some fundamental, essential property of a thing makes it the kind of thing it is, and essentialism was central to the Aristotelian and Scholastic views of knowledge against which modern science developed in large part as a critical reaction. Essentialism in biology, more specifically, was delivered its death blow by the triumph of Darwinism, and the consequent recognition that an organism might belong to a quite different kind from its ancestors, and that variation rather than uniformity was the norm for a biological kind. In a biology premised on variation and change, there is no reason to expect any unique answer to questions about how organisms should be grouped together, and a fortiori, there is no reason to expect science to provide such answers.
… It may be that excessive scientism, whether among lexicographers, high school science teachers, or just regular folk, will continue to favor a continuing convergence between scientific and ordinary language taxonomies. If this is the case, it does not reflect a gradual Peircean convergence on some objective reality but, rather, the hegemonic power of one, sometimes imperialistic, method of knowledge production. Perhaps for most folk in the West such imperialism is relatively harmless; for most of us urban and suburban folk what we call organisms doesn’t matter very much.
… Nevertheless, there are reasons for resisting, or at least pointing out, this imperialism. With regard to its effects on Western culture, the main such reason is simply to resist the excesses of scientism. The achievements and successes of science are amply evident, but it is also important that human culture has aims and projects that are distinct from and incommensurable with those of science, and science does not hold the answer to every question of human interest. … Especially in relation to cultures with more regular and direct interaction with nature, we would do well to explore thoroughly the basis and function of such classifications before criticizing them for their non-convergence on our own scientific categories. Once again, the perception of the value of Western science will hardly be enhanced by insisting on unsubstantiated claims to insight where this is not to be had.
… the classifications favored by science are distinguished from the rest not by their superior objectivity but simply by the specific, though various, goals that characterize scientific investigation of nature.
… finally, as well as misrepresenting in detail the scope of biological discovery, the denial that whales are fish propagates, in my view, bad philosophy. It reflects the assumption that inclusion of one kind within another can only reflect a positioning of the subordinate kind within a unique hierarchy of kinds, the hierarchy gradually being disclosed by biological science. But in fact there are many such partially overlapping and intersecting hierarchies. This situation would be usefully highlighted by the much more perspicuous claim that whales (and dolphins and porpoises) were mammalian fish.
Regrettably, I have had to admit that whales are not fish, for the sufficient reason that almost everyone in our culture, ignoring the partially excellent advice of Goldsmith, agrees not to call them so. Most folk assume, I take it, that biological kinds either fully include one another or are wholly disjoint. And thus since certainly not all mammals are fish (and vice versa), none can be. It would be futile and ridiculous of me to attempt a campaign for the reinstatement of whales into the realm of fish. Nevertheless, the recognition that there is no good reason for their having been excluded from this category would be salutary.