Species

The Nineteenth Century, a Period of Change 121

associationist psychology is evident here, but also the Lockean idea of general terms
as creations of the mind to collate past experiences. It is unclear if this was, as Hull
suggests, the core of his objections to Darwin’s theory of evolution, but at the least
his philosophical adherence to logical essentialism certainly played a part in it.
I nt erest i ngly, after Darwin, a kind of essentialism regarding species was
espoused by Jevons,^25 but he makes it clear that this refers to diagnostic species;
that is, to classes of definitions of objects. He states that in a “natural” system of
classification, all

arrangements which serve any purpose at all must be more or less natural, because, if

closely enough scrutinised, they will involve more resemblances than those whereby

the class was defined^26

and thus they are inductive groups, based, in living beings, on “inherited resem-
blances,” such that the “arrangement ... would display the genealogical descent of
every form from the original life germ.”^27 Therefore, diagnostic essences are cor-
relations that are causally important. Jevons follows Porphyry in treating Species
as a predicable.^28 Sir William Hamilton, however, in his Lectures on Metaphysics,
notes that we begin classification in “vague and confused” generalities, from which
we refine our discriminations of things until we end, rather than commence, with the
individuals, so that the genealogy of our knowledge is rather the history of how we
came to know them than the history of how they came to be.^29 Species are a product
of our getting to know things, giving an analogy:

We perceive an object approaching from a distance. At first we do not know whether

it be a living or an inanimate thing. By degrees we become aware that it is an animal,

but of what kind—whether man or beast—we are as yet not able to determine. It con-

tinues to advance, we discover it to be a quadruped, but of what species we cannot

yet say. At length, we perceive it is a horse, and again, after a season, we find that it is

Bucephalus.^30

The use of the notions of genus and species in logical discussions seems to have
petered out with the introduction of set theory and formal logic by Venn, Cantor,
Peirce, Frege, and others toward the end of the nineteenth century, especially around
1870–1878 in the case of Cantor. Where the inclusion of a species in a genus and of
lower species in that species was the mainstay of classificatory logic prior to this
development, now the talk was of sets and subsets. Moreover, the introduction of set
theory itself seemed to override the older approach of diairesis, or top-down divi-
sion. Sets could be defined from larger sets by division, or by aggregation of smaller
sets. Even more radical was the distinction between intensional and extensional

(^25) Jevons 1878, 710 –713.
(^26) Jevons, op. cit., 680.
(^27) Loc. cit., italics original.
(^28) Jevons, op. cit., 698.
(^29) Hamilton et al. 1874, Vol. 2, Lect. XXXVII.
(^30) Hamilton, op. cit., 334.