ABSTRACT
Second, as the question is usually put, it asks for a single, simple, final distinction, and for one that confers praise. This results, I suppose, from the old tradition of defining things by genus and differentia; that is, by naming first the class to which each thing belongs, and then the characteristic which marks it out from other members of that class. This rather hopeful scheme is supposed to enable us to find a formula stating the essence of each thing (or rather of each natural kind). And the differentia ought indeed to be in some way the thing’s characteristic excellence, its central function-since that, and not just some chance quality (as in “featherless biped”), is useful in helping us to place it sensibly, in telling us, therefore, what it is really like. The old, more or less Aristotelian, definition of man as a rational animal follows this pattern and is its best-known example. 1
Now most people today would with good reason reject this scheme as too ambitious to use outside the human scene. We cannot expect (they would agree) that things not made by man will necessarily have an essence we can grasp and a simple characteristic excellence we can see the point of. Evaluating snails from the human point of view is a fallible process and should be taken as such. We can certainly find marks that will help us to classify and understand them. But we had better not claim that by doing so we have finally expressed their true nature in a simple formula.
