Scientists are often assumed to be obsessed by definitions. After all, if you cannot precisely define a concept, say what a planet is, or what a biological species is, you literally don’t know what you are talking about, and how can you then possibly do science using that very same concept? And yet, the practice of science is very different, and to a surprising extent does not seem to depend on definitions of its objects of study.

Take the recent brouhaha concerning whether Pluto should be considered a planet or a different kind of celestial object (a captured asteroid perhaps, or a “planetoid,” whatever that may be). My colleague Neil deGrasse Tyson is a strong advocate of the Pluto-is-not-a-planet school, for which he has been chastised even by Jon Stewart. That idea won the day, and now the solar system only sports eight planets. But as I’ve argued in a Skeptical Inquirer column, the question is academic in the strictest sense of the word: it does not matter in the least to astronomy or planetology whether one officially designates Pluto as a planet or as a lesser entity.

The interesting scientific fact is that Pluto has several distinctive characteristics from the other eight planets (most notably the shape and angle of its orbit around the Sun), characteristics that require an explanation that is different from the one found to be satisfactory in the case of the “other” planets.

The issue is even more complex, and the technical discussions more acrimonious, in the case of biological species. Biologists and philosophers of science have been debating it for decades, and the resultant literature is voluminous, intricate, and largely inconclusive. (A few years ago I suggested that this is because “species” is a particular kind of concept identified by philosopher of language Ludwig Wittgenstein, and known usually as “family resemblance” or “cluster” concept: it does not admit of a simple definition in terms of a small set of necessary and sufficient conditions. Rather, it is fuzzy, made of a number of conceptual strands that intersect in a complex fashion.) As in the case of planets, however, this lack of an agreed upon definition has not stopped biologists from studying species, their characteristics, and even their modes of origin (i.e., speciation processes). How is this possible?

It turns out that there are two very different ways of thinking about “definitions,” ways that were beginning to be parsed by Socrates and Plato back in ancient Greece. Many of the early Socratic dialogues (those that more likely represent Socrates’ actual thinking, as opposed to using the figure of Socrates as a mouthpiece for the more mature Platonic philosophy) have at their core a discussion aiming at defining a particular term.

So, for instance, Euthyphro is about the definition of piety, Meno is about courage, Protagoras about goodness, and Republic 1 about justice. In all of them, Socrates and his companions pretty soon find themselves engaged in a heated discussion along the lines of “what is X?” which they take to be central to making progress in whatever endeavor they happen to be pursuing.

A naive reading of these dialogues has brought some people to talk about the so-called “Socratic fallacy,” the idea that one cannot say anything about X unless one can precisely define X. This is obviously not true. Not only, as I mentioned before, can biologists happily proceed with studying species even though they don’t agree on a definition of species, but in every day life as well we talk about all sorts of things (skyscrapers, baldness, porn) even though we would be hard pressed to give an exact definition of those same things (what’s the minimum height of a building that qualifies it being a skyscraper? When is it exactly that a man turns from having sparse hair to being bald? And of course there is the famous quip by American Supreme Court Justice Potter Stewart that he could not precisely define pornography, but he knew it when he saw it).

Besides, Socrates was too smart to fall into that sort of trap. Indeed, the way he went about examining concepts clearly shows that he did not commit the “Socratic fallacy.” The philosopher was famous for his method of “elenchus,” that is showing that someone’s understanding of an idea was mistaken based on the production of counter-examples that did not fit that person’s original explanation of the idea. For instance, in Euthyphro, the character that gives name to the dialogue at first claims that piety is to do whatever the gods wish. But Socrates quickly forces him to admit that that can’t be right, because in that case piety would simply be an arbitrary construct backed up only by (supernatural) force, not grounded in any inherent goodness.

There must be something else to it, which Euthyphro is obviously missing. Socrates could not use the method of elenchus if he really thought that one cannot begin to talk about X unless one has a precise definition of X: in that case, how could one even think of a counterexample? A counterexample to what?

What Socrates is after, then, is not a precise a priori definition of a given concept, but rather a theory of the extent and applicability of that concept. This isn’t something that can be arrived at by simply consulting a dictionary, but it requires thoughtful philosophical investigation. The very same thing is true of modern science: not only is the absence of a precise definition no embarrassment to scientists, it is that very search for a theory of X (planets, species) that defines what science actually is.

That search is also where scientists and philosophers talk to each other across the divide between the two cultures: whenever a philosopher identifies a problem with the way a scientist deploys a particular concept, the philosopher has uncovered a legitimate area for further conceptual (i.e., philosophical) and/or empirical (i.e., scientific) inquiry.

For the scientist to shrug off the suggestion and dismiss it as “just semantic” is then a naive mistake, one made out of sheer intellectual snobbism, and therefore unbecoming to a true intellectual.