Natural kind  

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In analytic philosophy, the term natural kind identifies a grouping of singular objects that always share particular qualities, whether or not humans know either the objects or qualities. A natural kind labels a "real" structure in the natural world, not an artificial result of human reasoning, by which scholars try to explain common sense knowledge with scientific knowledge.

Those who believe in natural kinds debate their structure and how it can be known. The structure of kinds is assumed to be constant, despite worldly experience being in constant flux. According to tradition, natural kinds can be known in two forms: as knowledge of general ideas about kinds, acquired by deductive reasoning, and as knowledge of observable examples of kinds, known by inductive reasoning.

Results of deductive and inductive reasoning are stated in propositions that express relations among objects and their qualities. For example: "All water is H2O." "All men are mortal." "All crows are black." "All light-skinned humans are more intelligent than dark-skinned humans." "All abortions are murder."

Believers in natural kinds claim such propositions describe objects and related qualities that exist. But they recognize that inductive propositions about "all" of a kind are problematic It is impossible for anyone to observe "all" of anything. Drawing conclusions about all members of a kind after observing a small sample appears illogical. But this kind of reasoning appears to be necessary, and often "works," meaning proves to be accurate.

Scientific disciplines frequently divide the particulars they study into kinds and theorize about those kinds. To say that a kind is natural is to say that it corresponds to a grouping that reflects the structure of the natural world rather than the interests and actions of human beings. We tend to assume that science is often successful in revealing these kinds; it is a corrolary of scientific realism that when all goes well the classifications and taxonomies employed by science correspond to the real kinds in nature. The existence of these real and independent kinds of things is held to justify our scientific inferences and practices.

This article identifies some contentious issues surrounding natural kinds. It presents a minority view expressed by John Dewey that natural kinds are a false conclusion of faulty instrumental reasoning. It presents several majority views expressed by four more recent scholars--W.V.O. Quine, Hilary Kornblith, Hasok Chang, and Rasmus Winther--that natural kinds are real and important scientific facts.


John Dewey (1859-1952)

In 1938, John Dewey published Logic: The Theory of Inquiry. He there explained how modern scientists practice induction and deduction, and why they have no use for natural kinds.

The philosophical issue is how humans can infer one kind of thing from another. The traditional answer grew out of Aristotle's assertion that humans state the kinds they know in two kinds of general propositions. Existential kinds--known by observation--are stated as "generic" general relations. Conceptual kinds--known by reasoning--are stated as "universal" general relations.

Dewey argued that modern scientists do not follow Aristotle in treating inductive and deductive propositions as facts already known about nature's stable structure. They treat propositions as intermediate steps in inquiry, hypotheses about processes displaying stable patterns. Aristotle's generic and universal propositions have become tools of inquiry rather than results.

"Propositions as such are ... provisional, intermediate and instrumental. Since their subject-matter concerns two kinds of means, material and procedural, they are of two main categories: (1) Existential [generic means, known by induction], referring directly to actual conditions, as determined by experimental observation, and (2) ideational or conceptual [universal means, known by deduction], consisting of interrelated meanings, which are non-existential in content ... but which are applicable to existence through the operations they represent as possibilities."

Modern induction starts with a question to be answered or a problem to be solved. It identifies problematic subject-matter and seeks potentially relevant qualities and conditions. Generic existential data thus identified are reformulated--stated abstractly as if-then universal relations capable of serving as answers or solutions.

"No grounded generic propositions can be formed save as they are the products of the performance of operations indicated as possible by universal propositions. The problem of inference is, accordingly, to discriminate and conjoin those qualities [kinds] of existential material that serve as distinguishing traits (inclusively and exclusively) of a determinate kind."

Dewey described these abstract steps concretely with the example of the kind "morning dew." From antiquity, the common sense belief had been that "All dew is a kind of rain," meaning dew drops fall. By the early 1800s the curious absence of rain and the growth of understanding led scientists to examine new qualities. Functional qualities of bodies [kinds] as solid-liquid-gas, and operational constants of conduction and radiation, led to new inductive hypotheses "directly suggested by this subject-matter, not by any data [kinds] previously observable. .... There were certain [existential] conditions postulated in the content of the new [non-existential] conception about dew, and it had to be determined whether these conditions were satisfied in the observable facts of the case.

After demonstrating that dew could be formed by these generic existential phenomena, and not by other phenomena, the universal hypothesis arose that dew forms following established laws of temperature and pressure. "The outstanding conclusion is that inductive procedures are those which prepare existential material so that it has convincing evidential weight with respect to an inferred generalization. Existential data are not pre-known natural kinds, but become conceptual statements of "natural" processes.

"Objects and qualities [kinds] as they naturally present themselves or as they are "given," are not only not the data of science but constitute the most direct and important obstacles to formation of those ideas and hypotheses that are genuinely relevant and effective."
"We are brought to the conclusion that it is modes of active response which are the ground of generality of logical form, not the existential immediate qualities of that which is responded to."
"Dewey concluded that nature is not a collection of natural kinds, but rather of reliable processes discoverable by competent induction and deduction. Assuming kinds to be given leads to the error of assuming that conceptual universal propositions can serve as evidence for generic propositions; observed consequences affirm imagined causes. "For an 'inference' that is not grounded in the evidential nature of the material from which it is drawn is not an inference. It is a more or less wild guess." Modern induction is not a guess about natural kinds, but a means to create instrumental understanding."

Willard van Orman Quine (1908-2000)

In 1969, W.V.O.Quine brought the term "natural kind" into contemporary analytic philosophy with an essay bearing that title. His opening paragraph laid out his approach in three parts. First, it questioned the logical and scientific legitimacy of reasoning inductively from a few samples to all members of a kind. "What tends to confirm an induction?" Second, it assumed that colors are inductively-identified natural kinds, despite some logical puzzles that can be solved: non-black ravens and green-blue emeralds. Finally, it suggested that human psychological structure can explain the illogical success of induction: "an innate flair that we have for natural kinds.

Quine believed the ancient method of induction can identify kinds. It groups experiences by recognizing similar objects, all possessing shared traits: "... each [observed] black raven tends to confirm the law [universal proposition] that all ravens are black ..." Observing shared generic traits warrants the inductive universal prediction that future experience will confirm the sharing: "And every reasonable [universal] expectation depends on resemblance of [generic] circumstances, together with our tendency to expect similar causes to have similar effects." "The notion of a kind and the notion of similarity or resemblance seem to be variants or adaptations of a single [universal] notion. Similarity is immediately definable in terms of kind; for things are similar when they are two of a kind."

Quine interpreted humanity's "innate flair for natural kinds" as innate criteria for judging degrees of similarity separating kinds that exist independently of human knowledge. These criteria work instrumentally when applied inductively: "... why does our innate subjective spacing [grouping] of [existential] qualities accord so well with the functionally relevant [universal] groupings in nature as to make our inductions tend to come out right?"

He admitted that generalizing after observing a few similarities is scientifically and logically unjustified. The number and degrees of similarities and differences humans experience are infinite. But the method is justified by its instrumental success in revealing natural kinds. The "problem of induction" is how humans "should stand better than random or coin-tossing chances of coming out right when we predict by inductions which are based on our innate, scientifically unjustified similarity standards."

"A standard of similarity is in some sense innate. This point is not against empiricism; it is a commonplace of behavioral psychology. A response to a red circle, if it is rewarded, will be elicited by a pink eclipse more readily than by a blue triangle; the red circle resembles the pink ellipse more than the blue triangle. Without some such prior spacing of qualities, we could never acquire a [classification] habit; all stimuli would be equally alike and equally different."

Quine credited human ability to recognize colors as natural kinds to their evolutionary role in human survival--distinguishing safe from poisonous kinds of food. He recognized that modern science often judges color similarities to be superficial, but denied that replacing obvious similarities by abstract universal similarities makes natural kinds any less permanent and important. The human brain's capacity to recognize abstract kinds joins the brain's capacity to recognize existential similarities.

"Credit is due to man's inveterate ingenuity, or human sapience, for having worked around the blinding dazzle of color vision and found the more significant regularities elsewhere. evidently natural selection has dealt with the conflict [between visible and invisible similarities] by endowing man doubly: with both a color-slanted quality space and the ingenuity to rise above it. He has risen above it by developing modified systems of kinds, hence modified similarity standards for scientific purposes. By the [inductive] trial-and-error process of theorizing he has regrouped things into new kinds which prove to lend themselves to many inductions better than the old."
"A man's judgments of similarity do and should depend on his theory [universal propositions], on his beliefs; but similarity itself, what the man's judgments purport to be judgments of, [is] an objective relation in the world. It belongs in the [generic] subject matter not of our [universal] theory ... about the world, but of our [universal] theory of the [generic] world itself. Such would be the acceptable and reputable sort of similarity concept, if it could be defined."

Quine argued that the success of innate and learned criteria for judging degrees of similarity, applied inductively to small samples of kinds, constitutes evidence of the existence of natural kinds; consequences affirm precedents. His reasoning continues to provoke philosophical debates.

Hilary Kornblith ( )

In 1993 Hilary Kornblith published a review of debates about natural kinds since Quine had launched that epistemological project a quarter-century earlier. He evaluated Quine's "picture of natural knowledge" as natural kinds, along with subsequent refinements.

Quine's original assumption that discovering knowledge of mind-independent reality depends on inductive reasoning, despite its being illogical, continued to be acceptable. His further assumption that success of inductive reasoning confirms both the existence of natural kinds and the legitimacy of the method also continued unquestioned, by Kornblith himself and scholars at large.

"I argue that natural kinds make inductive knowledge of the world possible because the clustering of properties characteristic of natural kinds makes inferences from the presence of some of these properties to the presence of others reliable. Were it not for the existence of natural kinds and the causal structure they require, any attempt to infer the existence of some properties from the presence of others would be no more than quixotic; reliable inductive inference would be impossible. The [generic] causal structure of the world as exhibited in [universal] natural kinds thus provides the natural ground of inductive inference."

Quine's assumption of an innate human psychological process--"standard of similarity," "subjective spacing of qualities"--also remained unquestioned. Kornbluth provided new labels for the necessary cognitive qualities: "native processes of belief acquisition," "the structure of human conceptual representation," "native inferential processes," "reasonably accurate detectors of covariation." "To my mind, the primary case to be made for the view that our [universal] psychological processes dovetail with the [generic] causal structure of the world comes ... from the success of science.

Kornbluth denied that this logic makes human classifications the same as mind-independent classifications: "The categories of modern science, of course, are not innate." But he offered no explanation of how the two can be distinguished.

"If the scientific categories of mature sciences did not correspond, at least approximately, to real kinds in nature, but instead merely grouped objects together on the basis of salient observable properties which somehow answer to our interests, it would be utterly miraculous that inductions using these scientific categories tend to issue in accurate predictions. Inductive inference can only work ... if there is something in nature binding together the [generic] properties which we use to identify kinds. .... Unobservables [universal propositions] are then postulated to explain the constant conjunction of observable properties."

Hence "raven" and "black" refer to concepts that match natural kinds because any black raven constitutes at least some evidence that all ravens are black. But "nonblack" and "nonraven" are not, because a nonblack nonraven (an extremely wide category) is not evidence that all nonblack things are nonravens. Nelson Goodman's problem predicate "grue", meaning "observed before 1 January 2050 and blue or observed after 1 January 2050 and green", turns out to be inappropriate because it does not denote a natural kind, according to Quine. He argued that kind-hood was logically primitive: it could not be reduced non-trivially to any other relation among individuals. See also: New riddle of induction#Quine.

Cultural artifacts

Cultural artifacts are not generally considered natural kinds. As one author puts it, "they never stop changing, and terms that designate them constitute only what Wittgenstein called 'family resemblance predicates'" (ibid, p.169). This point is more disputed; John McDowell has extensively argued that this opposition between "culture" and "nature" cannot be clearly formulated and that in any case it ought to lead us to construe cultural products not as unnatural, but as, adopting Aristotle's terminology, a kind of "second nature."

Philosophical issues

There is considerable debate in analytic philosophy about whether there are any natural kinds at all, and if so, what they are.

Philosophers of biology argue about whether biological species, like the bald eagle (Haliaeetus leucocephalus), are natural kinds; even such familiar species as bird, cat, and dog cannot be established as natural types since any plausible definitions of those species leaves the classification of some animals ambiguous.

Others debate whether races, sexes, or sexual orientations are natural kinds, or rather, to what extent they can be given the wide and continuous variety of race- and gender-related qualities. Meteorologists classify a number of different kinds of clouds, but it is not clear whether they are really different kinds, or whether those groups merely reflect the classifying interests of human beings – in order for them to classify as natural types, some clearly discrete circumstances would have to be shown to produce them in clearly distinguishable ways.

Practical utility

The argument, however, is not so much about whether natural kinds exist or whether any of our current concepts succeed in defining true natural kinds; but, rather, why certain concepts result in more reliable theories and successful interactions with the world.

One possible explanation is that it is because some concepts are better approximations of natural types than others, even if no perfectly discrete natural kinds exist and we can only hope to define concepts that classify reality with fewer marginal cases than others. In other words, the concept of the "natural type" may itself not refer to a natural type, but rather to an unattainable ideal that paradoxically leads to actions and decisions that achieve the best possible outcomes for human beings.

Linguistic issues

Language is often drawn into the debate as a complicating factor.

Those who contest that language heavily shapes abstract thought, or even that it makes abstract thought possible, tend to argue that the coincidental properties of one's native language(s) will inevitably influence one's concepts and interfere with the recognition of natural kinds. Words that reference ideas, such as "love" and "honor" — or metaphysical concepts, such as "karma" and "noumenon" — may be conceived of as not even attempting to refer to natural kinds as consistent delineations of objective reality.

At the other extreme, a word may claim to refer to a natural kind in a dogmatic and infallible manner, leaving those who understand the word (as such) unable to recognize or understand evidence that defies their linguistic categories; for example, members of a culture that defines democracy as inherently good may be unable or unwilling to understand the fact that Athens voted to commit genocide on several occasions. The linguistic argument is that it is the way in which cultures inevitably bias words such as "democracy" (or, for that matter, "genocide") that makes us ultimately unable to apprehend natural types even if they did exist. Such approaches are associated with Hermeneutics and the family of subjectivist philosophies generally.

See also

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