A few years back, as you may recall, there was some debate in scientific circles about whether Pluto should be classified as a planet—one that prompted some public consternation (and ironic T-shirts) among members of the general public dismayed to be told that, contra what they’d learned in middle school, there were only eight planets in the solar system after all.

But the reclassification wasn’t really the result of new information we learned about Pluto. Rather, it turned out that there were rather a lot more Pluto-sized masses in the general vicinity of our sun than we’d previously believed.  And so, as routinely happens in the history of science, it was decided that the term “planet” needed to be given a rather more precise and rigorous definition than had previously been deployed.  Astronomers settled on three criteria: A planet was an object (a) in orbit around the sun, (b) massive enough to be shaped (approximately) into a sphere by its own gravity, which also (c) had “cleared the neighborhood” of its orbit, meaning it had either absorbed or flung away other objects of similar size. Objects like Pluto, which met criteria (a) and (b), but failed on criterion (c), were instead classified as “dwarf planets.”

As quite a few prominent astronomers proposed, the definition could have been more lax, classifying everything that met the first two conditions a “planet”—in which case Pluto would have remained in the club, along with several other bodies. Moreover, some ambiguities in the definition remain, meaning future distinctions and clarifications might be necessary. Now, a child might hear about this debate and ask whether the astronomers had gotten it right: Were we now sure Pluto wasn’t a planet, or could it turn out to be after all?  But an adult would understand there’s not really anything  to be “right” about here—no genuine fact of the matter apart from how the relevant experts agree to use language.

Once a sufficiently precise definition is agreed upon, of course, it is an empirical question when and whether it is satisfied: Scientists can discover a new object that does or does not meet the new definition, and their classifications can be objectively correct or incorrect, as far as the precision of the definition allows, depending on the accuracy of their measurements.

But it would be misleading to say science had “learned” or “discovered” that Pluto is not a planet.  They had reached a consensus—albeit a somewhat unstable one—not about the facts of the external world, but about how to use a particular term.  That consensus could be more or less useful for a variety of purposes, elegant or inelegant, but unlike (say) a consensus view about whether the Earth will warm more than 3 degrees Celsius over the next century, it could not in any meaningful sense be “correct” or “incorrect,” because it is not a consensus about anything external to itself.

Which brings us, in a roundabout way, to an exchange between Michael Brendan Dougherty, Philip Bump, and Ryan Cooper over the question of “when life begins” and whether we should care about this when debating the morality of abortion.  Dougherty sees Bump and Cooper as making two distinct arguments, neither of which he finds compelling: Bump is arguing that “when life begins” is something of a “philosophical question” and that “implantation” of the zygote is at least as valid an answer as “at conception.”  Cooper will happily allow that a zygote (and therefore a fetus) is a “human life” in biological terms, but denies that this should be equated with moral or legal personhood.  I typically prefer Cooper’s framing myself, since it seems less likely to get a discussion hung up on irrelevancies, but really these are the same point—or at any rate, closely related ones—seen from a different angle.

When does life begin? Or, to more precisely identify the real question at issue here—because, of course, “life” is really a process that extends throughout the reproductive process rather than some magical quiddity that materializes at a particular instant—when does a new living member of a particular species come into existence? Doherty thinks biology “doesn’t give us options,” and rattles off some reasons why “at conception” is supposed to clearly be the right answer:

After the fusion of sperm and egg, the resulting zygote has unique human DNA from which we can deduce the identity of its biological parents. It begins the process of cell division, and it has a metabolic action that will not end until it dies, whether that is in a few days because it never implants on the uterine wall, or years later in a gruesome fishing accident, or a century later in a hospital room filled with beloved grandchildren.

The first thing to note is that this is wrong for a reason Dougherty alludes to in the next paragraph without seeming to recognize why it’s significant.  Until gastrulation, the zygote does not necessarily have “unique” human DNA—nor, indeed, is “it” even a determinate number of “its”: It can implant and continue growing into a single adult human, or into identical twins, triplets, or n-tuplets.  So at the moment of conception, how many lives begin? Does one new human come into existence and then divide into triplets? Or should we say all three somehow come into existence simultaneously as a potentiality, prior to division? (Are possible triplets murdered if the cell fails to divide?)

The point of raising this little puzzle is less to elicit an answer than to make it obvious that any answer is, like the status of Pluto, a matter of making choices about language rather than discovering some important fact about the world. None of the list of properties Dougherty ascribes to the zygote require us to say: “This is when a new human organism comes into being.”  We could rattle off a different list of biological processes that correspond to the unfertilized ovum, or the beginning or end of “conception” (actually a process lasting some 20 hours), or implantation, or the beginning of brain activity, or viability and declare those the “obvious” demarcation point at which a new organism exists.  Having picked some set of criteria as the most useful line of demarcation, of course, it will be  a matter of empirical fact when those criteria are satisfied.

Science can’t “tell” us which are the “right” criteria, however; scientists can only pick a set of conventions they find it useful to agree upon so that everyone can use terms consistently.    No external facts about the world, for that matter, requires us to use the same umbrella term, “life,” to describe the processes of animals, plants, and eukaryotes alike—or to draw it narrowly so as to exclude viruses, crystalline growths, or self-replicating automata we might construct in the future.

As the bizarre taxonomy in the Celestial Emporium of Benevolent Knowledge imagined by Jorge Luis Borges reminds us, even our “scientific” terms do not somehow cleave the world along its preexisting seams.  We draw up schemes of classification that serve our particular purposes, but should not imagine that utility for one purpose somehow makes that schema true in some objective or transcendent way.   We individuate species by the test of whether member organisms can interbreed, which turns out to be handy for generalizing about the trajectory of their evolution—”reproductive compatibility” turns out to be a very useful way to carve up the world—but to imagine that some further ontological shift happens at the moment of speciation is pure magical thinking—and, of course, for asexually reproducing organisms like bacteria, entirely different criteria must be employed. If our biological categories have any moral significance, it has to be because of (some of) the underlying properties we use to define that category are themselves significant (or highly correlated with morally significant properties), not merely because it suits us to group them under a particular label for independent, non-moral purposes.

Which brings us to Cooper’s point: Having stipulated some particular criteria for when we’re prepared to say “a distinct human (or animal) organism exists,” it may well be that “science tells us” those criteria are satisfied at some particular point.  But no interesting moral conclusions follow  from our taxonomic choice.  Those categories will, of course, correlate with properties we find morally significant—adult biological humans will routinely have properties that make it appropriate to respect an array of moral and political rights—but mistaking the correlation for the thing itself is just a biological form of cargo cult fetishism.  If, for instance, we gained the ability to resurrect biologically dead humans after days or weeks, only an astonishingly obtuse thinker would suggest that this biological category somehow resolved the question of when it was morally permissible to cremate a (resurrectable) corpse. It is no surprise that we naturally conflate biological humanity and moral personhood—in the overwhelming majority of contexts we encounter, they overlap—but laundering this fact about our linguistic habituation into a “common intuition” is a recipe for circularity and confusion when the point is precisely to determine where that overlap begins.