Kinds and the Wave Theory of Lighti
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JED Z. BUCHWALD* KINDS AND THE WAVE THEORY OF LIGHTI 1. Kinds, Instruments and the History of Science FOR SEVERAL decades many historians of science have not felt comfortable with philosophers of science, because contemporary philosophy has not often seemed to provide much that would be useful in historical practice. History wants pragmatic value from philosophy. Philosophy has until recently been unable to provide much of it. "Until recently" seems to imply that philosophy of science is or is about to become useful. To the extent that normative concerns persist, philosophy remains without much importance to historians.' It must instead try to penetrate what characterizes science in a way that captures something historically essential about it, something that can for that reason be put to practical use by historians in their work. I believe that something like this may soon come into being; it is, moreover, something that in a vastly less formal way many historians have long used. Over the last decade or so Tom Kuhn has developed a new approach, stimulated originally by the concept of kinds, to resolve issues that emerged from his Structure of Scientific Revolutions, in particular those that orbited about the concept of incommensurability.' Caught initially by John Stuart Mill's theory of real kinds in the System of Logic, and which other philoso phers had developed into a theory of natural kinds, Kuhn eventually found these to be unsatisfactory and prefers now to write of unqualified kinds. Ian Hacking, on reading in particular Kuhn's Shearman Lectures, has proposed a tInstitute for the History and Philosophy of Science and Technology, Victoria College, University of Toronto, Toronto, Ontario, Canada M5S IK7. Recei\'ed 16 JUlie 1991; ill revised/orm 26 September 1991. IBoth Tom Kuhn and Ian Hacking have been more than generous in providing me with their recent thoughts on the subject of kinds. Their work on it !l1otivates and underlies my consider ations here, which do not claim to provide philosophical novelty. My own interests here are more practical than theirs, in that I am concerned with the doctrine of kinds almost entirely for its usefulness in understanding the behaviour of groups of scientists, and in particular their creation and use of instruments. 2Although I am going to be discussing something that does look as though it might be used to make normative decisions about past science, I do not think that it can be because it concerns only one aspect of science history - an important, indeed central, one to be sure, but not one that provides a Royal Road, as it were, to good science. See note 43 for a brief discussion. lSee Kuhn's 'The Presence of Past Science', The Shearman Memorial Leetflre, London: Univer sity College, London, 1987'; 'Possible Worlds in History of Science', Proceedings of Nobel Symposium 65, ed. S. Allen (Berlin: de Gruyter, 1989); 'An Historian's Theory of Meaning', talk to Cognitive Science Colloquium, University of California, Los Angeles, 26 April 1990; 'The Road Since Structure', Presidential Address to the Philosophy of Science Association, 1990. SllId. Hist. Phil. Sci .. Vol. 23, No. I, pp. 39-74, 1992. 0039-3681/92 $5.00+0.00 Printed in Great Britain © 1992. Pergamon Press pic 39 SHIPS 23:1-0 40 Studies ill History alld Pizilosopizy of Sciellce philosophical explication of Kuhn's still-evolving theory.' On reading, first, a draft of Hacking's discussion and then Kuhn's recent work, I realized that one had here the possibility of formalizing and thereby clarifying something that has long formed a part of my own work on the history of electromagnetism and optics, namely the frequent failure of groups of scientists to understand the work of other groups: the problem, that is, of incommensurability. Although there are broad and important differences of opinion between Kuhn and Hacking about kinds, these do not matter for my purposes here, which will rely only on the core idea. In order to avoid these controversial issues I will follow Hacking in writing of "scientific kinds", without, however, intending to imply anything more than that these are the kinds that are deployed by scientists. I will here remain neutral concerning the important and difficult question between Kuhn and Hacking as to whether all kinds are taxonomic. The essential concept, which I will not elaborate in any detail, is this. Scientific practice (at least) is characterized by the separation of whatever scientists working in a particular area investigate into special groups, or scientific killds. Physicists might examine kinds of light, or kinds of electric conductors or, in general, any set of kinds that are together thought to constitute a related group. The kinds that form such a scientific group differ from categories in general in (at least) one essential respect: namely, that kinds can completely contain other kinds, but partial overlap among kinds is forbidden. To take a deceptively simple example, consider kinds of electric conductors as they were conceived in, say, the 1850s. Two large classes seemed to exhaust the universe of conductors: metals and electrolytic solutions. They differed in a number of respects, but the essential one concerned their chemical behaviour while carrying a current. Metals did not decompose into their chemical constituents; electrolytes did. Metals as a class did not have known sub-classes, since they were distinguished among one another solely by the value of a single parameter, their conductivity. Electrolytes had many sub-classes, reflecting .. their particular chemical structures.' As a kind, then, 'conductors' contained two sub-kinds, metals and electrolytes, one of which in turn contained other sub-kinds. Containment is a natural, essential characteristic of this structure. But partial overlap among the classes must not occur, because otherwise they would lose their meaning as scientifically meaningful kinds. No conductor can be both a metal and an electrolyte; no electrolyte can decompose chemically when carrying a current into more than one set of constituents. 41. Hacking, 'Working in a New World: The Taxonomic Solulion', for a volume containing essays in honour of T. S, Kuhn read at the Massachusetts Institute of Technology in May 1990, forthcoming from MIT Press, ed. P. Horwich. sChemical structure was not important for metals because it was precisely their failure to decompose when carrying a current that distinguished them as a class from electrolytes. Kinds and the Wave Theory of Light 41 In other words, nothing which is embraced by a given class within a particular group of scientific kinds can be both an a-thing and a b-thing, where a and b are group kinds, unless aU a-things are b-Ihings or vice versa. AU decomposable, current-carrying liquids are electrolytes, but there is no such thing as an electrolytic liquid that can decompose into one set of constituents when, say, carrying a large current, and into another set of constituents when carrying a small current. If there were such a thing then the entire kind structure for electrolytes, and possibly for conductors in general (even more radically, perhaps for chemistry itself), would have to be thoroughly re-worked. In more formal terms, if kinds are thought of as sets of objects,' then an object can be a member of more than one set in a given group of sets (call this group the object's family in respect to a particular area of investi gation)' only if every set that it is a member of either subsumes, or is subsumed by, another member of the family - scientific sets have no mutual connection at all or else they nest within one another like Russian babushka dolls (better, like a strange kind of babushka in which each doll can contain many other dolls that are immediately visible when it is opened, and so on until dolls that cannot be opened are reached). 'For present purposes no distinction will be made between objects in the ordinary sense, such as chairs or even tachyons, and objects like waves of light. even though one commonly says that objects of the fonner kind may produce objects of the lattcr kind as effects. Such a relationship would presumably take its place in an appropriate taxonomy, in which objects of the latter kind sort objects of the former kind. 7A particular object might fall into kinds that belong to completely different areas of inquiry. A liquid might for example be considered solely in respect of its viscosity for determining what kind of hydrodynamic entity it is; the same liquid might be considered in respect of its effect on the phase of reflected light in considering what kind of optical object it is. These two groupings of kinds - hydrodynamic and optical- might very well have nothing at all to do with one another, in which case one might have a hydrodynamic object of type h that has reflection properties of type r.. and another hydrodynamic object of the same type h that nevertheless has optical properties of type rt>. (In fact, during the late nineteenth century one did have something like this, since aniline dyes are rather viscous, like most solutions, but they are nearly unique in exhibiting marked anomalous dispersion in the visible spectrum, with accompanying oddities in reflection spectra.) What one cannot have is a'hydrodynamic object that falls simultaneously into hydrodynamic kinds that are not nested. Of course it is certainly possible that optical and hydrodynamic behaviour might eventually be brought together (by, e.g. linking molecular structure to both hydrodynamic and to optical properties), but if this does happen then the kinds will have to be reconstructed to prevent overlap. Hacking pointed out to me an important, and I think related, objection to the non-overlap condition (see Hacking, op.