Biological Species & Natural Kinds

If I have one big idea it is that one can understand Platonic and Aristotelian ontology and metaphysics better (or perhaps at all) by comparing them with modern evolutionary biology and vice versa.  I think I began to understand Aristotle much better, after decades of reading the Physics and the Metaphysics, when I realized that many contemporary debates in the philosophy of biology were recapitulating problems already present in Aristotle. 

Two days ago, I grabbed a book off my self on the way to the toilet.  Some of my biological processes seem to require reading material.  Darwinism and Philosophy [ed. Vittorio Hösle and Christian Illies (2005)] included an essay by Michael T. Ghiselin.  The latter is most famous for his answer to one of the great questions of modern biology: what is a species?  Is it a natural kind, like oxygen in the periodic table of elements?  Is it a group of (potentially) interbreeding organisms like elk?  Is it merely a customary grouping, like red pieces of laundry?  Each of these answers has its proponents. 

Ghiselin’s answer is that a species in an individual.  If that makes no obvious sense, you are in good company.  It made no sense to me until, sitting on porcelain and doing God’s work, I realized that Aristotle had joined the conversation.  By the time I finished the paperwork and returned to my desk, I was convinced that I understood Ghiselin’s argument and I was convinced that he was right. 

An individual is something that is located in space and time and, for that reason, has a history.  The Garmin fitness tracker on my left wrist is an individual; this one is mine, not somebody else’s.  The same is true of my wrist and me.  I had a beginning in time and my being traces a continuous path from that time and place to now.  The same is true of Homo neanderthalensis.  That species had a beginning, spread out across portions of the globe, and came to an end. 

Natural kinds are not located in time and space the same way.  Gold is gold wherever it occurs, whether it was dug up in California or discovered on Mars or produced in a supercollider.  As opposed to individual lumps of gold, gold has no history; it has only essential properties. 

Aristotle covered this same ground.  There are things that have properties-horses for example-and there are the properties that such things have-shape, weight, and species forms.  The things that have properties would seem to be the things that are real.  A horse can change color (age happens, even to Seabiscuit) and yet remain a horse; its color exists on in the horse and the color ceases to exist when the color has changed. 

This suggests one answer to the question: what is most genuinely real?  For the purposes of biology, at least, it is the individual organism.  From the horse one can abstract downward to its properties (color and size), its matter (meat and bone), and its parts (limbs and organs).  Likewise, one can abstract upwards to its species form, which links it to the other horses.  All of these are abstractions because they exist only in analysis; they depend for their reality on this here animal. 

Aristotle might have rested with this had he not inherited the central problem of Greek philosophy.  What is most real should be what is most knowable.  It is not clear that I can know anything about individual organisms.  Is she short or tall, young or old, alive or not.  On the other hand, I can know that horses are mammalian quadrupeds in a rather more secure way. 

Ghiselin’s view that species are individuals seems to address this problem.  If species are not eternal, they are not knowable in the same sense that triangles are knowable.  They are at least more knowable that a particular horse.  His account points, however, toward a solution to the problem. 

Ghiselin does not deny that the species as natural kind account points to something real.  The species horse is an emphatically real thing: there are lots of horses.  The natural kinds horse, mammal, etc., are real in so far as horses really are horses and mammals.  He just insists that natural kinds of this sort are properties of individual species. 

A good indication that we are on the right track is that our account to solve problems that once seemed too difficult.  I was raised, intellectually speaking, in a hotbed of natural rights theorists.  Under the guidance of Harry V. Jaffa, we took the Declaration of Independence very seriously.  In that I have not wavered. 

The Declaration speaks of the Laws of Nature and of Nature’s God.  Inferred from such laws is the self-evident truth that all men are created equal in so far as they enjoy certain unalienable rights.  Here I understand “men” to mean human beings.  My colleagues who reject evolutionary biology do so on the grounds that undermines confidence in the existence of a stable human nature that can bear such properties as unalienable rights. 

In the evolutionary scheme, species seem to differ not in kind but only in degree.  For example, lions and tigers can mate and produce ligers, hybrids that are not capable of having offspring.  This means that the two species share a large percentage of their DNA but not enough to produce a lineage.  So lions and tigers differ, genetically, only in degree.  Species as individuals emerged from common ancestors and so differ only in their unique history and genomic families.  From that point of view, the critics appear to be correct. 

Not so fast.  Individual species also present real differences in natural kind.  A particular lion may be so much a tiger, genetically speaking, and he may be bigger than more aggressive than another male lion when he squares off on the field of reproductive competition.  He is, however, no more a lion than any one of his competitors.  Socrates, Aristotle insists, is no more a man than any of his interlocutors. 

To bring this back to the Declaration, allow me to indulge in a little science fiction.  Suppose we encounter living organisms on another world.  Assume that these organisms emerged historically through the same evolutionary processes as life on earth.  We will find there no organisms that belong to the same species as those on our planet, at least in Ghiselin’s terms.  None of those species will have a common history with any of our species.  Yet the fact that we recognize them as living organisms means that they do share something vitally important.  The living organism is a natural kind.

Now suppose that we encounter Vulcans or something of that sort.  Green blood and distinct biological history notwithstanding, they are capable of symbolic communication and moral responsibility.  They can recognize mutual obligations between themselves and us newcomers.  They are capable of making contracts.  Are they “men” as the Declaration means that term?  Of course.

Human beings are certain kinds of beings.  Our species has its unique biological history, just as do individuals and populations of individuals within that larger individual that is our species.  While we are diverse in many respects, no son or daughter of humankind is more an organism, or a mammal, or a human being than any other.  I find the distinction between species as individuals and natural kinds to be worth thinking about.

What is the human thing? 2

I have been reading Aristotle’s Categories tonight.  I first read it as an undergraduate philosophy major at the University of Arizona.  As always when I return to this master I am again astonished by the power and durability of his thought.  Twenty-three centuries after his death, he continues to illuminate and be illuminated by the efforts of those who try to become wise by asking one question after another. 

I confess that I did not know until tonight that his word for categories is the same word that is frequently used for an accusation.  To categorize someone, in common language Greek, was to accuse him of a crime.  So to identify Donald Trump as a man is to accuse him of something.  This is the sort of thing that Martin Heidegger would have written a whole, incomprehensible book about. 

The Categories begins with a basic fact about language.  We use some words as homonyms, which sound identical but mean entirely different things.  Think of baseball bat and vampire bat.  I got that example from a fellow grad student Kevin Long.  Sometimes we use words as synonyms, as when we speak of sexually reproducing organisms and asexually reproducing organisms.  Finally, we sometimes use words as paronyms, as when we say that someone who does politics is a politician.  That last term, unfortunately, never made its way into English. 

From that point of departure, Aristotle dives deep into metaphysics.  Consider this statement: “the leaf on the tree is green”.  Green is what we say about the leaf.  The leaf is what we say green about.  Aristotle uses that distinction as the basis of what is more or less real.  The things that are most real are the things that are never said about anything else.  We might say that a tree leafs out, but we never say that this here tree “that here leafs”.  The leaf is a real thing which can be green and fresh or brown and withered.  The latter terms are meaningful and true only if they describe something real. 

The term for real thing-ousia in the Greek-is usually translated as substance, and for a very good reason.  The same words in both languages indicate both a substratum that undergoes change (the iron that goes from black and cold to red and hot in the blacksmith’s forge) and the property of a “man of substance.”  See note on Heidegger above. 

For Aristotle, the only genuine, “primary” substances are individual things.  His examples are always organic: this here human being or this here horse.  His real things are the real things in the common sense meaning of the words, the things we can see and touch.  Aristotle is presumably arguing with his equally famous teacher, Plato, who taught that apparently more abstract things like beauty, truth, and goodness were the real things. 

Having made this point, Aristotle immediately qualifies it in Plato’s direction.  He does so by making a distinction between primary substance (this here horse) and secondary substances like horse and animal.  The latter are secondary (and hence not quite genuine) substances because they can be said about something else.  So one can say that Ken Blanchard is a human being and an animal but one never says that this is a Ken Blanchard or that this Ken Blanchards about anything but yours truly. 

We do, however, speak about species and genus (horse and animal in his examples) the same way that we speak about individual creatures, as when Aristotle says that the human being is the political animal.  In this case, the human being is what political animal is said about.  This gives Aristotle a way to stack the candidates for genuine substance in order of reality.  Species (horse) is more real than genus (animal) because the latter can be meaningfully predicated of the former (a horse is an animal), yet less real than Seabiscuit, which cannot be predicated of anything.  

The problems that Aristotle is addressing here continue to haunt biology to this day.  What is the biological substance?  David Hull and Michael Ghiselin have argued that a species is not a class but an individual.  I am the particular person I am not because I look like my father but because my mother and father begat me.  Dogs are dogs not because they have this or that definitive trait but because they were sired by other dogs. 

While I don’t necessarily buy into this, I think they are onto something.  Evolutionary biology fleshes out Aristotle’s thinking by extending it backward into organic time.  Organisms branched off into plants and animals.  Animals split into distinct species.  Yet all of this depends, at every actual moment, on actual organisms, then and there, surviving, being fruitful and multiplying.  Aristotle’s candidates for substance are ranked as they are in evolutionary history.  Pretty good for a guy who never looked into a microscope. 

I will close this post with a little quote from Aristotle (Categories sec. 5). 

It seems that no substance is more or less [what it is]… I mean that no substance can admit of degrees in itself.  For example, the same substance, a man, cannot be more or less a man as compared to another.  One man cannot be more a man than another, in the same way that one that one white thing can be whiter than another white thing or one beautiful thing more beautiful than others. 

Take that, my Southern ancestors!  Aristotle recognized, a good two thousand years before Jefferson, that all men are created equal.  Darwin may or may not have been motivated towards his work by an opposition to slavery, in fact confirmed Aristotle’s reasoning.  No human being is more human or whiter than another.