Aristotle on exceptions in biology

Petter Sandstad

University of Rostock

Prima facie, one would think that exceptions should disprove a rule (viz. a universal predication). E.g. if one in Euclidean geometry were to find a triangle with internal angles not equal to two right (which is an impossibility), that would disprove the rule that the internal angles of a triangle equals two right. However, that does not seem to be the case in inter alia biology. There it is not uncommon that an exception does not disprove a rule. It might not prove the rule, but it can give further evidence in favour of the rule. Only this seems to be paradoxical. Here I am not thinking of cases of prevention, where one removes a precondition, or a stimulus, or adds an interfering agent. E.g. not the case where a bird cannot fly because its wings has been clipped, or because it is sleeping, or because it has been drugged, etc. These exceptions present no major difficulty for the general rule. It is not absurd to say that the bird still has the disposition to fly even though it has been drugged and cannot currently fly. Or for that matter, that it would have had the disposition if it had not been prevented. The cases I am thinking of are e.g. species of birds that cannot fly simpliciter, e.g. the dodo. Aristotle was well aware of this general problem, and I think his solution convincing. Briefly put, his answer is that exceptions like this do not disprove the rule, provided that the exception is explainable in terms of the form (viz. nature or way of life) that also explains the rule. In the case of birds, the disposition to fly varies in degrees chiefly with the environment and source of food (cf. PA IV.12., 693b28-694a11; & Leunissen 210:129ff.) And a difference in degree can easily become a difference in kind (cf. John Cook Wilson). Thus the general rule that birds fly is explained by the form of «birdness», yet that form also explains the exception viz. why some species of birds cannot fly. To elaborate on this solution, I connect it with Aristotle’s causa formalis, specifically to the discussion in the Posterior Analytics (cf. I. 4-5, and II. 14-18) on universal predications and on how to make such a predication at the correct level (viz. his doctrine of the proton katholou). The sound universal predication thus identifies the correct form/kind (viz. species/genus) to which the attribute belongs per se. In a non-exact science (e.g. biology) the complexity of the forms involved allows for exceptions, because the exception itself is explained by this form. In other words, even though the attribute is universally predicated to a kind, variances in degrees of this attribute within the forms of the kind (the species of the genus) even allows for the form to completely lack the attribute (e.g. flying and the dodo). Although this seems paradoxical, it is in fact not.