Natural Kinds

It appears to us that things in the natural world divide into different kinds. The most obvious examples come from biology. Cats are clearly distinct from mice; while both kinds show variation, all the cats are more similar to one another than they are to any mouse. We see different kinds of tree and different kinds of lichen. These are kinds that are apparent to any reasonably careful observer. Other kinds seem to be revealed by science. The chemical revolution gave us new ideas about what it is to be a chemical element, and in the subsequent decades many dozens of these different basic chemical kinds were revealed. The following century uncovered a multiplicity of different kinds of fundamental physical particle. While at a different set of scales, geologists distinguish different kinds of rock, meteorologists distinguish different kinds of weather system, and astronomers distinguish different kinds of galaxy. The philosophical questions that natural kinds generate can themselves be categorized into three types as they relate to metaphysics, philosophy of language, and philosophy of science. (It should be noted nonetheless that discussion of these questions quite rightly overlaps these fields. Consequently, works listed in this article may be relevant to further sections in addition to the ones they are listed under.) Here are examples of the philosophical questions surrounding natural kinds. Does the world itself genuinely have a structure of objective natural kinds, so that there are natural divisions of things by kinds? And do our actual natural classifications match those kinds? Or are what we take to be kinds merely the product of a particular non-objective perspective? How does a natural kind (or belonging to a natural kind) differ from other natural properties? In virtue of what do things group themselves into kinds? How is kind membership determined—for example, by necessary and sufficient conditions or by something else? Do, indeed, natural kinds have essences? Are there entities that are the natural kinds? Do our natural kind terms refer to such entities rather as names refer to objects? Is there a non-trivial way of spelling out the idea of rigid designation for natural kinds terms? Do the semantics of natural kind terms vindicate essentialism about kinds? Does what we discover about kinds in the special sciences (e.g., concerning chemical substances and biological species) support or undermine philosophical conceptions of kinds?