For a project on the use of images as a source of data in biology, I found myself thinking about the phrase “carving nature at the joints.” To a biologist the implication of this phrase is clear enough, because it captures a common goal of biologists, and indeed of natural philosophers going back to antiquity. For example, in biology the goal may be to identify the natural divisions separating biological phenomena into groups of the same kind–we see this in the enterprise of classifying organisms into species and higher level taxonomic groups, or recognizing how different genes below to the same “gene family.” Or the goal may be to to identify the functional subdivisions of the organism that make up the integrated whole–the organs that perform different functions, the tissues that make up the organs, the cells, cellular compartments, and genes that define function on more basic organismal levels. Either way this “carving” is an essential first step in biology: part of the description of the phenomena to be studied–a statement about how things are put together, and what things are alike in spite of superficial differences.
In the case of understanding the subdivisions of complex organisms, the carving up of phenotype is sometimes fairly easy to achieve. The bones of the vertebrate skeleton are easily distinguished from one another at the joints (the root of the metaphor, of course) or at the the suture lines dividing joining the individual bones of the skull or pelvis. For other phenomena, it is not so easy. The difficulty is especially pronounced for behavior, which unfolds in a more or less continuous stream in which subunits, and their boundaries, beginnings, and endings, are hard to parse.
But this is about tracking down the history of a metaphor, rather than the biological problems themselves.
The most direct and widely cited use of this metaphor comes from a book on Evolutionary Psychology by Leda Cosmides and John Tooby. They say, “”Knowledge of adaptive function is necessary for carving nature at the joints.” Evolutionary psychologists try to explain the evolution of mental phenomena by proposing how natural selection may have tailored them to solve particular challenges to survival and reproduction. The assumption of the field, as in the approach to animal behavior pioneered by Darwin, is that the psyche can be segmented into separately evolving subunits in the way that we describe the elaboration of mammalian phalanges into wing struts in bats, or into a hooved foot in horses.
More recently, ”Carving Nature At The Joints” was used as the title of recent book by Keim et al. (eds)(2011) that collects papers by philosophers on the topic of “natural kinds” in metaphysics and the philosophy of science.
What about the original source of the metaphor? For that we have to go back to Phaedrus, by Plato. In this dialog, Plato has Socrates explaining two principles of rhetoric “the essence of which it would be gratifying to learn, if art could teach it.” One of these principles is “That of dividing things again by classes, where the natural joints are, and not trying to break any part, after the manner of a bad carver.” So there you have it–the metaphor in its full glory. That’s the second of the two important principles offered by Socrates, by the way. The first principle is “That of perceiving and bringing together in one idea the scattered particulars, that one may make clear by definition the particular thing which he wishes to explain.” Note that you really need both of these principles to capture the problems biologists face in describing nature. The carving problem corresponds to finding “the ‘natural’ suture lines for evolutionary dynamics” (to use a formulation developed by Richard Lewontin (1979) in identifying a pitfall in the study of adaptation by natural connection). The bringing-together problem is about recognizing when seemingly disparate things (bats and cows, for example) belong in the same category (mammals).
How do images help? That’s a story still to tell.
Lewontin, R C 1979. Sociobiology as an adaptationist program. Behavioral Science 24 (1): 5-14.