[Plato's] most famous metaphor for the reality of universals [a view explained in the linked essay] was to say that real universals "cut nature at its joints" (Phaedrus 265d-266a). He compares the task of definition to the job of being a butcher. The clumsy butcher just hacks things up in any old way, but the expert butcher deftly slices the animal at its natural joints, neatly separating naturally distinct segments of the animal. He gave an extremely influential example of this method of definition in Sophist 218d-221c.A different resource that might be useful to you is the Stanford Encyclopedia of Philosophy's entry on boundaries, if you're specifically thinking about geographic joints.
This is the original inspiration for what is now called "taxonomy." The Greek word "taxis" refers to the arrangement of a group of soldiers. Biological taxonomy is the study of the arrangement of living organisms into the categories: Kingdom, Phylum, Class, Order, Family, Genus, Species. Plato's idea here is that the various species are real features of the world. Tobermory my cat really is a cat (Felis catus). By distinguishing cats from dogs we "carve nature at its joints" and are recognizing a real distinction which exists in nature. This is the realist view of species.
It seems to be generally agreed that Plato said it first, Phaedrus 265d-266a .
posted by methylsalicylate at 5:50 AM on March 6, 2007