In a recent paper, Nicholas Smith responds to Michael Bruno and J.M. Fritzman’s response to him about group belief. Smith, contra his critics, argues that two things’ performing the same function might be no reason at all to think that they’re the same kind of thing.1 This is a post about the work that kinds are doing in his argument.
Considering the category mousetrap, Smith argues that there is more to it than just performing the function. He makes two points:
First, there are two primary kinds of mousetraps: lethal traps that kill mice and live traps which contain them so that they can be transported elsewhere. Smith writes, “Both sorts of devices are called ‘mousetraps,’ but those in the market for a mousetrap might well prefer one of these sorts of devices over another, precisely on the ground that the actual function of each kind of ‘mousetrap’ is importantly different.”
Second, something might do the job of systematically killing mice without thereby being a lethal mousetrap. He writes, “Even though my cats are remarkably good at catching and killing mice, it nonetheless seems to me that it would be a dreadful metaphysical error to conclude from that fact that cats and mousetraps and the same kinds of thing.”
The upshot of these arguments would be that being a mousetrap is more than just performing the mousetrap function— because the function is (1) ambiguous and (2) insufficient for being a mousetrap.
Both arguments reflect a confusion about what kinds are or could be.
Regarding the first: The fact that there are two different kinds of mousetrap doesn’t mean that mousetrap itself isn’t a legitimate kind, anymore than sexual dimorphism shows that Mus musculus (the common house mouse) isn’t a legitimate kind. The general category mousetrap which includes both lethal and live varieties might, for all that’s been said, be perfectly legitimate.
Regarding the second: A lethal mousetrap does more than just kill mice. It is also an engineered device which can be manufactured. These are other functional relations it has which, were we to include them in a richer specification, would distinguish (for example) a Victor Easy Set Mouse Trap from Tibbles the cat.
However, it seems likely that at least some breeds of cat were selectively bred as mousers. So the Victor Easy Set and Tibbles are both household objects developed with the common purpose of killing mice, after all. A householder might well face a choice between having a cat and putting out snapping traps. Insofar as this decision is made around considerations of mouse control, the two are competitors and (to that extent) the same kind of thing.
This underscores that the naturalness of a kind is relative to a domain of phenomena.2 For biology, cats and mousetraps aren’t members of the same kind— but mousetraps aren’t among the biological phenomena at all. For home economics, perhaps yes. And for mouse epidemiology, they are both notable members of the kind mortal threat. Which is to say, there are many legitimate kinds that include both cats are mousetraps. It would be a dreadful metaphysical error to deny this.3