Copyright © 1999 Massachusetts Institute of Technology
Some systems of classification are merely conventional: they divide a population of objects into kinds, and the principles by which objects are categorized are designed to answer to some specific purpose. There is no antecedently correct or incorrect way to categorize various objects apart from the purposes to which the system of classification will be put. Thus, for example, we divide the world into different time zones, and our purpose in so doing is to allow for coordination of activities in different locales, but there is no right or wrong way to draw the boundaries of time zones; there are merely more or less convenient ways that answer better or worse the concerns that led us to devise these categories in the first place. The view that all systems of categorization are merely conventional is called conventionalism about kinds.
Some systems of CATEGORIZATION, however, do not seem to be merely conventional. Rather, they attempt to draw conceptual boundaries that correspond to real distinctions in nature, boundaries which, in Plato's phrase, "cut nature at its joints." Thus, for example, the periodic table of elements seems not merely an arbitrary or convenient system of classification, a system that makes certain calculations or predictions easier; rather, it seems to describe real kinds in nature, kinds whose existence is not just a product of our classificatory activity. Kinds of this sort, which are not merely conventional, are called natural kinds. Those who believe that there are natural kinds, called realists about kinds, believe that it is part of the business of the various sciences to discover what the natural kinds are; scientific taxonomies, in this view, attempt to provide a proper account of these kinds.
The notion of a natural kind figures in to important questions in the methodology of the cognitive sciences, as well as work in COGNITIVE DEVELOPMENT, the psychology of reasoning, and COGNITIVE ANTHROPOLOGY.
According to conventionalism, many disputes about proper taxonomy in the sciences are misguided. Taxonomic systems, in this view, cannot themselves "get things right" or "get things wrong," although some will, of course, be more convenient than others. Disputes about taxonomy, in this view, do not involve genuine disagreement about substantive scientific questions. Realists, however, regard disputes about proper taxonomy in the various sciences as substantive. Consider, for example, the categorization of psychopathologies in the Diagnostic and Statistical Manual (1994). Those who think of psychodiagnostic categories as merely conventional will regard questions about categorization here as ones of convenience; a proper system of categorization is merely one that well serves the purposes for which it was designed. If, however, the various psychodiagnostic categories constitute a system of natural kinds, then there are substantive theoretical questions about the boundaries of those kinds. In this view, certain ways of drawing the boundaries among psychopathologies are simply mistaken, and not merely inconvenient or ill-suited to the purposes for which we have devised our classificatory scheme. More important, some systems of classification might be extremely useful for certain purposes without giving a proper account of the nature and boundaries of the items classified.
Those who believe that all classification is merely conventional thus see taxonomic disputes as shallow and the search for substantive theory to guide taxonomy as misguided; substantive scientific questions arise only after the choice of a taxonomic system. Realists about natural kinds, on the other hand, see taxonomic disputes as potentially important; a proper taxonomy must be guided by theoretical insight into the underlying causal structure of the phenomena under study. The dispute between conventionalists and realists is thus significant not only in issues concerning psychodiagnosis, but also in addressing taxonomic questions throughout the cognitive sciences.
The way in which questions about natural kinds influence taxonomic issues demonstrates the importance of this concept for methodological concerns in the cognitive sciences. Questions about natural kinds arise more directly, however, as well. First, natural kind CONCEPTS play a crucial role in inductive inference. Second, and relatedly, the acquisition of natural kind concepts plays an important role in cognitive development.
Natural kind concepts play an important role in successful inductive inference because members of a given natural kind tend to have many of their most fundamental properties in common. Thus, finding that one member of a kind has a certain property gives one reason for believing that others will share that property as well. This uniformity of natural kinds is part of what distinguishes them from arbitrarily specified classes of individuals, for in the case of arbitrary classes, noting that one member of the class has a certain property (other than the ones that are used to define the class) provides one with no reason at all to believe that other members of the class will share that property. Our ability to make successful inductive inferences thus depends on our ability to recognize natural kinds. Not surprisingly, many suggest an evolutionary basis for such a preestablished harmony between folk and scientific taxonomies. While no one believes that our native categories simply mirror those of the sciences, the suggestion here is that natural selection provides us with a starting point that approximately captures some of the real distinctions in nature, thereby allowing for the possibility of more elaborate and more accurate scientific taxonomies. Without some native help in identifying the real categories in nature, some have argued, we would be unable to develop accurate taxonomies at all.
It is for this reason that the acquisition of natural kind concepts is such an important intellectual achievement. Two developmental questions need to be separated here: (1) At what point are various natural kind concepts acquired? For example, when do children acquire the concept of a living thing, of an animal, of a being with mental states, and so on? (2) At what point do children acquire the concept of a natural kind itself? A good deal of work has been done on each of these questions. Although an explicitly articulated concept of a natural kind is certainly found in no children, and indeed, in few adults, the ways in which children classify objects and the ways in which they respond to the information that two objects are members of a single taxonomic category suggest that there is a strong tendency to view the world as having a structure that presupposes the existence of natural kinds. In particular, children do not tend to classify objects merely on the basis of their most obvious observable features, features that may be unrevealing of natural kind membership; and when children are told that two individuals are members of a single category, they tend to assume that these individuals will share many fundamental properties, even when the most obvious observable features of the objects differ. Some authors have suggested that these tendencies may be innate; this would help to explain the possibility of successful inductive inference by explaining the source of the human ability to identify kinds that support inductive generalizations. A tendency to view the world in terms of the structure required by natural kinds seems, at a minimum, to be an ability already in place early in cognitive development.
Relevant here too is work in cognitive anthropology. Atran's work (1990) on folk taxonomies reveals deep similarities in the ways in which different cultures divide up the biological world. More than this, these taxonomies have much more than just a passing resemblance to the more refined taxonomic categories of the biological sciences.
Not everyone is entirely optimistic about the possibility of filling in the details of the picture presented here. Some deny that the taxonomies of the different sciences have enough in common with one another to speak of them all as displaying a single structure, the structure of natural kinds. This skepticism has been fueled by a number of factors, including the recognition that the physical world is not deterministic, as well as the fact that the kinds of the biological world cross-cut one another. That scientific taxonomies are simply more messy than was once assumed has thus not only complicated the picture of natural kinds, but also made some doubt the very usefulness of the notion. Moreover, the similarity between folk taxonomies and the taxonomies of the various sciences differ substantially. The connection between our conceptual capacities and the causal structure of the world thus leaves a good deal to be discovered on all accounts.
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Copyright © 1999 Massachusetts Institute of Technology