Thomas Kuhn’s misunderstood relation to Kripke-Putnam essentialism

Introduction

Thomas Kuhn is well-known – or notorious -- for having invited philosophers of science to consider the possibility that there are senses in which ‘the world changes’ when science changes substantially enough. It was in part specifically in opposition to this idea of Kuhn’s that Saul Kripke and Hilary Putnam developed their now very-influential version of the idea of natural ‘essences’, ‘natural kinds’, which are unchanging through all ‘scientific revolutions’.

In his later years, labouring under the label of ‘Relativist’ in large part because of his words about ‘world changes’, Kuhn came to make what has been termed a ‘taxonomic turn’ – in other words, he himself turned to a conception of natural kinds, and suggested that perhaps a more perspicuous and less risky way of putting what happened in a scientific revolution was this: that there was a shift from one taxonomy of natural kinds to another, untranslatable into the first.

In our view, this later Kuhnian characterisation of scientific revolutions is important and useful. But the present paper does not seek to establish that. It has a narrower aim. It aims to defend Kuhn’s own later conception of natural kinds against certain common misunderstandings, by means of going onto the offensive. By means, that is, of seeking to explain just how devastating is Kuhn’s attack on the Kripke/Putnam [‘K/P’] doctrine, the now more-or-less standard doctrinal invocation of natural kinds to undermine Kuhnian ‘incommensurability’, and more besides.

Against rigid designation

The central question may be put thus: do the different categorial schemata, the different ‘taxonomies’, the different ‘lexicons’ which Kuhn identifies, merely glomm onto fully pre-existing objects or clusters? Putnam et al allege that they do. Kuhn, as we shall shortly see, suggests strong reasons for doubting that this Putnamian position is co-tenable with a decent understanding of the history and philosophy of science.

The key Kuhn paper addressing the issue just outlined is “Dubbing and Redubbing: The vulnerability of Rigid Designation”.[i] Kuhn’s intent and fighting spirit is clearly evident in the title here. He signals that, unlike the essentialists, he will concentrate not only on the ‘dubbing’ (naming, baptising) of kinds in the world, but on their redubbing, their re-classification – on revolutions, and their widespread nature and effects, in short. He further signals that he thinks that ‘rigid designation’ – the metaphysical idea at the heart of K/P essentialism – is vulnerable to Kuhnian objections, vulnerable to falling afoul of what Kuhn has said about serious conceptual change in science: about ‘scientific revolution’.[ii]

The underlying neo-incommensurabilist ‘position’ which Kuhn wishes to defend and build upon is stated clearly at the outset of his paper. It is to be noted that it wisely, modestly and crucially focuses upon the role not of the metaphysician but of the historian[iii]:

“To understand some body of past scientific belief, the historian must acquire a lexicon that here and there differs systematically from the one current in his or her own day. Only by using that older lexicon can historians accurately render certain of the statements that are basic to the science under scrutiny. Those statements are not accessible by means of a translation that uses the current lexicon, not even if it is expanded by the addition of selected terms from its predecessor.” (298)[iv]

Here is a pretty careful ‘linguistic’ (re-)formulation of an absolutely central message of Kuhn’s, throughout his career. Kuhn is suggesting clearly here that incommensurability be understood primarily as the (‘partial’) “untranslatability” of taxonomies of natural kinds across revolutions. The (double) scare-quotes are necessary because, “Incommensurability…equals untranslatability, but what incommensurability bars is not quite the activity of professional translators. Rather, it is a quasi-mechanical activity governed in full by a [Quinean translation] manual” (299).

The crucial sentences, for the purposes of trying to see what Kuhn is up to in discussing incommensurability under the heading of ‘world changes’, are perhaps the following:

“To possess a lexicon, a structured vocabulary, is to have access to the varied set of worlds which that lexicon can be used to describe. Different lexicons – those of different cultures or different historical periods, for example – give access to different sets of possible worlds, largely but never entirely overlapping.” (300) [v]

It is what Kuhn means to mean by such sentences as these that we shall now try to uncover.

The philosophical worries of Kuhn’s mainstream philosophical critics, facing Kuhn’s effort to establish the import of these changes of lexicon – radical alterations in taxonomy, in mode of classification or ‘ontology’ – might be expressed as follows: ‘There is only one world, and the Realist (unlike the Relativist or Idealist) can have coherent things to say about our deepening knowledge of that (one) world, because we (all of us, especially via scientists) are always in touch with the world. We are in touch with it through our naming of bits of it, and our growing knowledge about the nature of what we name. It may that at different times people had radically wrong ideas about the nature of the world – they may have thought, for example, that water was a primitive element – but, through being in contact with bits of the world (e.g. with water), and through naming it, they always had some kind of ‘basis’ to their claims. We can connect with them and what they said, because there are direct (albeit long) chains of connection – linguistic and (more generally) ‘causal’ – between their use of these words and our use of them. They may have ‘meant’ something very different by their words, but the reference of their words was just the same as our reference for the same words. Reference – and in particular a causal theory thereof – will settle the problem which meaning poses. Kuhn is wrong, because he thinks that incommensurability of meaning is important and deep. It is, in fact, completely shallow and unimportant, once one understands that the real reference for natural kind terms remains continuous over time and through ‘revolutions’.’

Kuhn made original and interesting replies to this kind of essentialism, making a strong case that the causal theory of reference will not solve the problems/challenges which ‘incommensurabilism’ poses.

The Kripke-Putnam[vi] essentialist story is usually run in two main versions. It speaks of the dependence of meaningful and consistent language-use on continuity of reference across SPACE, and also across TIME.

Reference across Space

This is the famous ‘Twin-Earth’ case – Putnam alleged that the meaning of ‘water’ would be different in a place (‘Twin-Earth’) if and where what superficially appeared to be water was actually a different chemical (‘XYZ’ for short, i.e. a substance with a different ‘essence’). The heart of Kuhn’s response is this: “The terms ‘XYZ’ and ‘H20’ are drawn from modern chemical theory, and that theory is incompatible with the existence of a substance with properties very nearly the same as water but described by an elaborate chemical formula. Such a substance would, among other things, be too heavy to evaporate at normal terrestrial temperatures. Its discovery would … demonstrate the presence of fundamental errors [anomalies] in the chemical theory that gives meanings to compound names like ‘H2O and the unabbreviated form of ‘XYZ’. Only within a differently structured lexicon [to that of modern, post-phlogistic, chemistry], one shaped to describe a very different sort of world, could one, without contradiction, describe the behaviour of ‘XYZ’ at all, and in that lexicon ‘H2O’ might no longer refer to water.” (310)

Is Kuhn merely fiddling here? Is he being a spoilsport, bringing in mere facts to bear on a philosopher’s charming speculations? No! He is raising a fundamental difficulty for the essentialist/referentialist view. He is saying: ‘If we try taking Putnam’s example seriously it turns out that Putnam doesn’t really offer up a tenable thought-experiment after all. Unless you are willing to fundamentally revise chemical theory, i.e. to alter it in a radical way, to turn the paradigm – in the full sense of the words, the taxonomy of chemical theory – upside down.’

Alexander Bird, in recent criticism of Kuhn on natural kinds, counters [vii] that Kuhn is muddying the waters, that his (Kuhn’s) objection can be finessed by means of switching to a time, perhaps the mid-nineteenth century, when we could know that XYZ is not water (and that water is H20), without knowing that XYZ cannot have the surface properties of water (unless modern chemistry is more or less completely wrong). But this just seems incoherent: it seems that now there no longer is a thought-experiment. For how are we supposed to know that water is H2O if we cannot rule out that some water is XYZ, if we cannot rule out XYZ as a starter in the game? And we cannot rule out the latter unless (e.g.) we know that XYZ cannot have the surface properties of water (unless, modern chemistry is more or less completely wrong, is in need of a fundamental taxonomic overhaul ). Unless and until one has a good scientific reason for insisting that water’s extension cannot include XYZ, one will not need to endorse Putnam’s ‘externalist’ conclusion. Kuhn’s point is: such good reason is ‘only’ in fact given by the taxonomic etc. structure of modern chemistry. The apparent discovery of XYZ would prompt not a reflection on the alleged truth of referentialist/externalist essentialism, but would rather herald a scientific revolution in chemistry.

In short, Putnam’s essentialism begs the question against Kuhn by ignoring the knock-on consequences of change in taxonomy, of change in the world-as-understood-by-science.

Time

Putnam suggests that ‘water’ referred to the same substance (H20) in 1750 as today, and that that’s an end of the matter. Kuhn, by contrast, is interested in the structure of (any given) Chemistry, and writes as follows:

“’H2O’ picks out samples not only of water but also of ice and steam. H2O can exist in all three sates of aggregation – solid, liquid and gaseous – and it is therefore not the same as water, at least not as picked out by the term ‘water’ in 1750.[viii] The difference in items referred to is, furthermore, by no means marginal, like that due to impurities for example. Whole categories of substance are involved, and their involvement is by no means accidental. In 1750 the primary differences between the species recognised by chemists were still more or less those between what are now called the states of aggregation. Water, in particular, was an elementary body of which liquidity was an essential property.’’ (311)

Kuhn then remarks: “This is not to suggest that modern science is incapable of picking out the stuff that people in 1750 (and most people still) label ‘water’. That term refers to liquid H2O. It should be described not simply as H2O but as close-packed H2O particles in rapid relative motion.” (312) Can the essentialist not acknowledge this point as a point about meaning, but still hold onto the essential referentialist conclusion? No, because “The lexicon required to label attributes like being-H2O or being-close-packed-particles-in-rapid-relative-motion is rich and systematic. No one can use any of the terms that it contains without being able to use a great many. And given that vocabulary, the problems of choosing essential properties arise again… Is deuterium hydrogen, for example, and is heavy water really water?” (312)

Essentialism can only answer these kinds of questions by ignoring the ‘systematicity’ that Kuhn is drawing our attention to, and instead by arbitrarily counting certain things as essential, others as accidental – a procedure surely inimical to a scientific approach, and making a mockery of the idea of natural kinds. Kuhn’s view is that we must understand the progress of science as periodically involving the junking of (at least parts of) taxonomies of natural kinds, and their replacement by others. ‘Rigid’ designation no longer seems quite to deserve the name – for, though scientific schemas do indeed (of course, and contra relativistic and sociologistic fantasies) describe/explain the world across time and space, they are themselves often destined, even in many of their fundamentals, not to have the permanency of what they are used to depict.

Bird again tries to counter Kuhn, by suggesting that Kuhn’s reasoning does not touch the ‘essence’ of Putnam’s conclusion that “in all possible worlds water consists (largely) of H2O.” [ix] The word “largely” here is interesting. Presumably, Bird is allowing what Kuhn also allows: that water with some impurities is still H2O. But what, for example, if those impurities are XYZ? What if those impurities turned out to be somewhat more numerous than expected; what if about 25 per cent of water on the Earth’s surface were ‘discovered’ to be XYZ (so that water is still ‘largely’ H2O)? Would essentialism still look so attractive, then?

It is only given our post-Lavoisieric framework that we are forced to see water as largely H20. Absent that framework, ‘water’-in-all-its-states is not necessarily viewed as a natural kind (as the quotes above from Kuhn make clear: liquidity was regarded as an essential property of water) -- and still less is H20. Kuhn is bringing talk of possible worlds, one might say (paraphrasing Wittgenstein), back from its metaphysical to a more everyday (i.e. everyday scientific) use. A taxonomy supplies a ‘set’ of possible worlds between which normal science goes on to choose. If something really threatens the taxonomy, we (imagining ourselves now into the position of scientists actually confronted by such an anomaly) cannot retreat to philosophers’ assurances about what all possible worlds must turn out to be like. Rather, sometimes, we must face the need to uproot fundamental assumptions about the set of possible worlds available to us and enabled for us by our taxonomy, our ‘ontology’, our thought-style.

Redubbing is then at least as important as dubbing; and, of course, in concert with Kuhn’s reasoned scepticism as to ‘Correspondencism’: progress through revolutions is not well-described as bringing us taxonomies which themselves come closer and closer to matching the universe’s ‘own’ taxonomy.

Conclusions

Kuhn’s conclusions are as follows: “[D]ubbing and the procedures that accompany it ordinarily do more than place the dubbed object together with other members of its kind. They also locate it with respect to other kinds, placing it not simply within a taxonomic category but within a taxonomic system. Only while that system endures do the names of the kinds it categorizes designate rigidly … Here and there the old and new lexicons embodied differently structured nonhomologous taxonomies, and statements involving terms from the region where the two differed were not translatable between them.” (315). This expresses clearly Kuhn’s constant insistence that ‘conceptual schemes’ are, because of their wholistic character, ‘incommensurable’, that one never really transplants an idea or expression from its context to a new one, for to remove an idea from its context is to denature it.

If we are not much mistaken, then, Kuhn’s taxonomic conception has enabled him to defend and re-specify the notion of incommensurability against the idea that it is reference, not meaning/use, that is overwhelmingly important. Kuhn’s ghost still lacks any reason to believe that referentialist essentialism undercuts his central arguments in SSR – and indeed, any reason to believe that such essentialism is even coherent, considered as a doctrine about anything remotely resembling our actual science. The actual relation of Kuhn to Kripke-Putnam essentialism, we have argued, is as follows: that Kuhn decisively undermines it, and leaves the field open instead for his own more ‘realistic’, deflationary way of thinking about ‘natural kinds’.