12 Species
dichotomously.^40 Aristotle was saying, as we would now describe it, that classi ca-
tion must always be cast in terms of proper sets and subsets. Partial inclusion is not
legitimate in a good classi cation, in effect because it does not make proper sets.
It is traditionally held that Aristotle was inconsistent in the way he used genos
and eidos between the logical and the biological writings^41 but Pellegrin, Balme, and
Lennox have shown otherwise.^42 In part, the problem arises because the common
view rests mainly on the post-medieval concepts that, we shall see, are derived out
of the later neo-Platonic revision of Aristotelian logic. Aristotle is only inconsistent
if understood to use the technical term eidos in the same way in the logical works as
he does in the context of biology, and he does not.
Phillip Sloan observed that the rst edition of this book failed to distinguish
between three senses of eidos:
the use of eidos as predicate of substance, as we encounter it in Categories; eidos as
form that is the universal grasped in perception and referred to in language; and eidos
as the individuated form-in-individual that in a living being is identi ed with psuche
as the principle of life and vital action.^43He is correct, and I thank him for the elucidation, but I note that this strengthens
rather than diminishes my interpretation. The so-called “Aristotelian essentialism”
interpretation of biological taxonomy transports the Categories sense into the natu-
ral historical sense.
Pellegrin says that Aristotle was not aiming to produce a biological taxonomy in
History of Animals. Instead, he was producing general classi cations, and animals
happened to be one domain in which he applied that method. What Aristotle treats as
genera and species do not answer directly to the modern, post-Linnaean, conceptions
of species, although this has sometimes been the default interpretation. We have seen
that for him a species is a group that is formed by differentiating a prior group formed
by a generic concept. Genera have essential predicates (or de nitions), and so do spe-
cies. In mae species happen to be indivisible except into individual things, that is all.
In this respect, biological species are no different from any other kind. Pellegrin says,
Aristotle thus conveys by the term genos the transmissible type that in our eyes char-
acterises the species, and by eidos the model that is actually transmitted in generation.
It would be necessary for these two terms to converge and become superimposed for
the modern concept of a species to be born. For Aristotle, the [biological] species did
not yet exist.^44(^40) Balme says that in Parts of Animals 1.2– 4 [Balme 1987, 19]:
(^) ... Aristotle concludes that diairesis can grasp the form if it is not used dichotomously as Plato used
it but by applying all the relevant differentiae to the genus simultaneously; after that he explains
the ways in which animal features should be compared so as to set up differentiae—by analogy
between kinds (genē), by the more-or-less as between forms comprised within a kind (eidē).
(^41) For example, Mandelbaum 1957.
(^42) See Wiener 2015 for an argument that Aristotle is in fact doing taxonomy in his biological writings.
(^43) Sloan 2013. See also the footnote to Peck’s translation of Generation of Animals 731b30–33 [Aristotle
1942 , 130–131n].
(^44) Pellegrin 1986, 110.