An essay in Nature recently, titled “A question of class” (by Jeffrey Parsons and Yair Wand) puts the case that classification is crucial to science and needs to be understood. They hold, as I do, that a poor understanding of classification – particularly of the concepts/words “class” and “category” – lead to unproductive and dangerous conclusions within science. But I don’t think they get there quite yet…
Classification – the act of putting things into classes – is something that every science does, ranging from elements and planets, to diseases, taxa and functions. The authors make the following, it seems to me entirely arbitrary, specification:
In science, classification is rarely related to survival so directly, yet the underlying principles are the same. Phenomena can be categorized in many ways on the basis of the properties they share. However, categories are useful only if they make it possible to infer further information, and only if they do so consistently and over a reasonable time period. To distinguish a general category from a more useful one with inferences, we call the latter a ‘class’.
Whereas a category simply reflects a repeating pattern of properties, a class additionally indicates that relationships exist between these properties, even if the mechanisms behind the relationships are unknown. For example, all the things in a room make up a category (they share the property of being in the room), but not necessarily a class; their presence in the room might not reflect some deeper underlying principle.
In fact, in traditional logic class and category are pretty much the same thing.* That you have put something into a class or category merely means that things that fall under it share some properties. These can be properties in the minds of the categoriser, or they can be shared properties out there in the world.
But something like this distinction is required, for reasons I will explain. We need to make a distinction between things gathered for conceptual reasons into an equivalence class, and things that just are in an equivalence class because they in fact have related properties. Rather than take up the confusing terminology of Parsons and Wand, I’ll rely on a prior distinction between type and taxon. A type is a class that is formed merely for conceptual purposes; that is, it exists in the head of the cogniser. A taxon is a class that is formed in virtue of sharing causal properties and historical origins.
Let’s consider their examples. A “planet” is defined by the International Astronomical Union in the recent redefinition excludes Pluto and other “planetoids” not because different causes are involved in their formation, but for purposes of convenience. The same causes are involved n the making of Jupiter and Pluto, but we need to have a classification scheme to deal with the increasing number of Trans-uranic objects of size.
But in the case of disease, we classify in various ways. Sometimes we know what the etiology of the disease is and classify it that way. Other times we merely have a group of similar diseases or symptoms, and classify together on that basis. In fields where the etiologies remain opaque, such as psychiatry, we only have classification by similarity, and this leads to trashcan categories – groups of everything else. The former are taxa. The latter are types. Types are formed by, as it were superficial similarities. Taxa are formed on the basis of large numbers of properties, where the underlying causal etiology is opaque, or by a few causal properties where the etiology is known. Consider the DSM IV. There is a category of “autistic spectrum disorders”, which includes full autism and Asperger’s Syndrome. So far as I can tell, these are quite distinct etiologies, and quite distinct symptomatologies. Putting them together for reasons of resemblance interferes with investigation of the causes and behaviours of these conditions.
In biology, there has been a longstanding issue about what counts as a biological classification, and the positions have been largely, since around 1975 or so, between those who use a similarity measure based on arbitrary choices of properties, called numerical taxonomy or phenetics (from the Greek phainero, to seem), those who hold that etiology (in this case genealogy of taxa) is sufficient, called phylogenetic systematics, or cladistics, and those who wish to use some more or less arbitrary mixture, called evolutionary systematics. The traditional Linnaean system can be applied to all of these approaches, and so does not represent anything methodologically deep, but rather only a matter of convention. However, those who use similarity metrics often defend Linnaean systematics, although not all who defend it are pheneticists. Some cladists prefer the Linnaean system as it “stores” in its descriptions a lot of information about the organisms, and wish merely to ensure that Linnaean groups are monophyletic; which is to say, that a Linnaean group of any rank does not exclude parts of an evolutionary branch based on similarities.
Philosopher Manfred Laublicher, of Arizona State University, has said that the difference between phenetics and cladistics is that in phenetics, the relationship is similarity, as I have said here, but the relationship between cladistic objects is one of identity. What does this mean? Take whales – a more derived form of mammals one cannot expect to find – no hindlimbs, no fur, often no teeth. And yet they are still mammals. Not “like” mammals, they are mammals. That is, they are “the same” as any other mammal. Even if they lost their mammary glands and started to bear young in egg sacs, they would still be mammals.
The reason why this identity matters is that even if whales lost some “key” characters of being mammals, like fur, milk, and live birth, they would still have a vast number of retained mammalian characters, and we could inductively project that they would, for example, share mammalian cytochrome C genes, and have evidence of genes that grow fur, mammary glands and hindlimbs still in their genomes. We could project their overall metabolism on a knowledge of their relatives. We would know that, if the nearest non-whale relatives of whales had a particular hormone or enzyme, so too would those whales.
But on a resemblance criterion, this is not inductively projectible. All that a class formed from resemblance allows you to infer is that the members, by definition, resemble each other in various ways you have set up by using the resemblance criterion. Imagine a class of warm blooded organisms (actually proposed by Julian Huxley as Homeothermia). If you know something is a member of that class, what can you infer? That it has warm blood, and nothing else. If you know something is a member of Cetacea, what do you know? Lots. You know its reproductive organs, internal physiology, skeletal elements, hearing structures, etc., no matter how derived or modified they may be.
Now there’s no reason why a type and a taxon cannot be coextensive, and a reason why many scientists are confused about this is that they often, but not always, are. Types are formed by the sharing of lots of superficial properties, and these are, in biology at any rate, often as much the result of shared evolutionary history as the deeper causal properties. But to investigate the natures of taxa, one must use etiological properties (such as homologies in biology), not superficial ones, no matter how useful the superficial ones are for diagnosis or identification.
Something that I think the oft-derided philosopher Alfred North Whitehead got very right is what he called the fallacy of misplaced concreteness. Here the mistake is to think that because you have a noun, there is a thing that the noun names. In taxonomy, scientists often mistake the name for the thing it names, or even conclude that the name of a class indicates there actually is a thing. This is a reason why types can mislead – have a type name and you soon start to think in terms of there being some natural thing the type names. But if the name is merely one of convenience, that is an error in inference and science. And this is the problem that Parsons and Wand do not quite reach.
* The real distinction between a class and a category is between a group formed by its members’ properties (a class), and a group formed by a predicate, or propositional attribute, or its members. In short, a class may be non-conceptual, while a category is conceptual and linguistic. It doesn’t matter here.