It seems biologists (and philosophers of biology) think that "Kitcher's
motivation for asserting that species are sets is to allow spatiotemporally
unrestricted groups of organisms to form species. That motivation, however,
is not substantiated by biological theory or practice." Species as sets
(see for context)

So it seems that this apparent prevalent opinion (according to Stanford
Encyclopedia of Philosophy) is rooted on the fact that defining them as set
is *difficult* for the scientist, because it is hard to compute and modify
the necessary and sufficient of their relationships and processes that
guarantee set membership, it seems that intensional definitions cannot the
capture "historical development" of distributed organisms that happen to be
taxonomically linked from tiem to time. So one define species as
Individuals and it seems easier to track their historical development. What
bothers me is that this seems, like you said, a matter of model-theoretic
reference and a kind of heuristic shortcut, an argument derived from
computational complexity.

On Thu, Oct 24, 2013 at 2:08 AM, meekerdb <> wrote:

> Ontological status is always within some model we have created.  So one
> can created models in which species are defined extenstionally and create
> different models in which they are defined intensionally.  So what?  They
> are both our creations to help understand the world.  Does one work better?
> Have more predictive power?  Do they imply some operational tests?  It is a
> waste of time to argue about essences and which one is really real.
> Brent
> On 10/23/2013 8:10 PM, Francisco Boni wrote:
>> Two apparently distinct ontological distinctions:
>> "After the development of set theory, however, a distinction of the
>> scholastics between intension, of sets that were circumscribed by
>> definitions, and extension, by member inclusion, was revived, and the
>> logical tradition of species was held to be a matter of intensional
>> definition. In a seminal summary of the traditional pre-set theoretic
>> logic of diairesis, or division, from the most general to the most
>> specific, H. W. Joseph (1916) made a clear distinction, as Whately had
>> 90 years earlier in 1826 (see below), between logical species and
>> “natural” species, but the developers of the essentialism story failed
>> to pick this up, and read him as saying that species of living things
>> were the same as the logical species (as discussed in Chung 2003,
>> Winsor 2001, 2003, 2006a)."
>> In a discussion of what counted as a kind in natural history, William
>> Whewell in his Philosophy of the Inductive Sciences (1840) gave a good
>> account of natural kinds as being types from which there were
>> deviations, although he treated species themselves as simple objects.
>> John Stuart Mill disagreed and asserted that natural kinds were
>> definable and had universal (causal) properties for all members (Mill
>> 2006).1 For Whewell, the type of the taxonomist was a natural kind;
>> for Mill, it was the element and the compound of chemistry and
>> physics. Still, I believe the weight of venerable history is on
>> Whewell’s side, not Mill’s, or to put it another way, that Mill’s
>> conception of natural kinds is not something that applies well to
>> historical sciences that are restricted to specific domains, like
>> natural history or taxonomy. And despite what we might think based on
>> the discussion of logic from Frege onwards, as late as the early 20th
>> century, for instance with John Venn (1866) and others, a natural kind
>> was indeed typically thought to be a kind of living beings, caused by
>> generation (Hacking 1991).
>> So much of the confusion about essences can be resolved if we do not
>> adopt the view that Mill introduced, that a real Kind must have a set
>> of necessary and sufficient properties. For Mill, a species would be a
>> natural kind (a phrase introduced by Venn, although he did not adopt
>> the Millian view regarding it; Mill just used the word Kind) if it had
>> some set of universally shared properties that made each organism a
>> member of it, rather like having a certain number of electrons,
>> positrons and neutrons makes each atom of an element that element. By
>> contrast, for Whewell, and for those taxonomists who he was accurately
>> describing in the Philosophy of the Inductive Sciences, to be a member
>> of some group, a species, a genus, and so on, is to be mostly like the
>> typical form, and to be something that can be supposed to share a
>> genealogy with that form. This sense of “essence” is something that, I
>> believe, is quite consistent with our present understanding of
>> genetics and populations; most members of most species share most
>> genes, but there are multiple genetic controls over some typical
>> traits, and some species have major genomic varieties. David Hull has
>> said that there is nothing so unusual or absurd in biology that some
>> species doesn’t have it somewhere or somewhen; I call this Hull’s
>> Rule. To be an essentialist in the world of Hull’s Rule means that you
>> cannot insist that taxa are going to always have some set of genetic
>> or other causes, but it doesn’t mean that you can’t say that taxa
>> mostly share causes. To be a taxon, I think, is to have some set of
>> general properties, the bulk of which any member will share.2 This is
>> sometimes called in philosophy the “family resemblance predicate”,
>> after Ludwig Wittgenstein’s example in the Philosophical
>> Investigations (Pigliucci 2003, Wittgenstein 1968), but over a century
>> before Wittgenstein, Whewell made just this case. Families resemble
>> each other because they share generative histories and hence
>> generative causes, but they share them typically.
>> The thesis known as the Individuality Thesis (Gayon 1996, Ghiselin
>> 1997, Hull 1978), in which species are considered to be not kinds, but
>> named objects that have a historical location, is a defense of
>> biology, and especially genetics, against the encroachment of Mill’s
>> notion of a Natural Kind. Sure, say the individualists, species and
>> other biological taxa are not Natural Kinds (as defined by Mill). The
>> only other metaphysical notion open to philosophers of taxonomy is
>> that of an Individual, a thing that exists in one time and one place
>> or region, and has a start and an ending. Hence, species are
>> Individuals. I cannot fault this logic – species clearly aren’t the
>> kinds of Kinds that Mill required, and they actually are historical
>> objects, so I have no objection to their being called Individuals; but
>> I do think they have “essences”, or, rather, typical developmental
>> systems and responses to typical environments; to preclude confusion,
>> let us call these developmental types.
>> There is a recondite argument in metaphysics as to whether individuals
>> in the metaphysical sense can have an essence – some say not, others
>> say they can. I won’t rehearse it here, as it is of interest only to a
>> certain kind of philosopher. However, there’s a much older sense,
>> Aristotle’s, that I think works well for individuals. Aristotle did
>> not have a technical term for essence, especially not for natural
>> objects, but instead he used the phrase “the-what-it-is-to-be”3. We
>> can usefully ask what it is to be a member of a species. Generally, it
>> is to share a genetic, developmental and ecological process. To be a
>> member of Homo sapiens is to have some majority of typical properties:
>> a certain number of chromosomes, genes, cell types, and ecological
>> resources that reliably result, most of the time, in the morphology of
>> the typical members of the species (Pigliucci 2003, Pigliucci and
>> Kaplan 2006). We can investigate this and list these properties
>> (although we must be careful not to call the first genome, such as
>> Craig Ventner’s, the “typical” genome); this is a matter of empirical
>> research. It is not a matter of definitions, it is a matter of finding
>> out what the developmental types are by sampling populations and
>> periods.
>> No philosopher of language who is inclined to the Millian kind of
>> Kinds or to linguistic or logical essentialism is going to be happy
>> with this, just as family resemblance predicates haven’t been greatly
>> liked by such philosophers in general (but see Gasking 1960), but it
>> is a sense of the essence of species that is at once compatible with
>> the biology, and consonant with the original history of the notion of
>> essences. Species can be metaphysical Individuals, and yet have
>> something mostly shared between its members, which can be granted the
>> honorific of “essence”, the what-it-is-to-be a member of that species,
>> its developmental type."
>> — John S. Wilkins, "What is a species? Essences and generation"
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