Species

122 Species

definitions of sets.^31 A species in the older logic had to be definable from the larger
genus. A set, however, could be described or defined. An extensionally defined
set is treated as isomorphic with another set, or, as Quine expresses it, “the law
of extensionality, which identifies sets whose members are the same”; intensions
(which Quine abhors) are specified by predicates, which “have attributes as their
‘intensions’ or meanings.”^32 Under the Aristotelian account, all species were inten-
sionally defined—this was the point of defining them by their essences. Now we
had aggregates that could be treated as synonymous merely by sharing all members,
irrespective of their essences. It was not immediately clear how this might apply to
the biological species problem, and it indeed took some time for it to be applied.
Similar essentialist accounts of biological species are also presented after Darwin
by several Roman Catholic authors, including a respected entomologist^33 and a
logician,^34 both Jesuits. A late example is a Dominican, Murray,^35 and a well-known
Canadian entomologist, William Thompson, another.^36 Thompson is particularly
interesting as he explicitly based his attack on Neo-Thomist philosophy. One might
conjecture that the essentialism of biological species bemoaned by Mayr and others is
in fact a reaction to Darwin and evolution (or at least Haeckel’s Romantic version of it)
rather than something he overcame, as the Received View has it. Perhaps, like Donne,
they found all coherence gone with a temporal and gradual transmutation of species
one into another. Perhaps it was the outworking of the revival, or rather invention, of
neo-Thomism after the First Vatican Council.^37 Amundson, however, has argued that
essentialism was invented in the 1840s by Hugh Edward Strickland, but this was a
purely taxonomic essentialism with no causal or empirical consequences.^38
Even so, some continued to use the older logical terminology and the Aristotelian
conceptions that underlay it well into the twentieth century, even if there were some
concessions to the new set theory. Husserl, for example, in 1913, writes in section 12
of his Ideas,

Every essence, whether it has content or is empty (and therefore purely logical), has its

proper place in a graded series of essences, in a graded series of generality and spec-

ificity. The series necessarily possesses two limits that never coalesce. Moving

downwards, we reach the lowest specific differences or, as we also say, the eidetic

(^31) The terms “intension” and “extension” are medieval, according to Joseph. Mill’s Logic introduced
the terms “connotation” and “denotation” [Joseph 1916, 146–155]. Leibniz is known to have also
contrasted the two terms.
(^32) Quine 1970, 67.
(^33) Wasma n n 1910.
(^34) Clarke 1895.
(^35) Murray 1955.
(^36) Thompson 1958, 1971, Thorpe 1973.
(^37) Proposed in conversation by Polly Winsor. I have my doubts, though“essence” is almost always
applied by the burgeoning Catholic intelligentsia to knowledge of the nature of God (e.g., by Cardinal
John Henry Newman), rather than to physical or material objects. The language was available, but it
appears to arise in biology much later. It is also a term in use by continental philosophers influenced
by Kant and Hegel; an obvious example is Marx; others include the existentialists such as Kierkegaard
(McOuat, pers. comm.). Nevertheless, I have no doubt that the Neo-Thomist revival was an influence
on this movement if only because it offered an alternative metaphysics to the Darwinian problem.
(^38) Amundson 2005, 51.