Quining Natural Kinds

One of the greatest philosophers of the second half of the twentieth century was Willard Van Orman Quine, the final true logical positivist. Quine was a student of Carnap, and his criticism helped move Carnap into a new phase in his career. Quine was, first and foremost, a technical philosopher who made strides in logic, set theory and related fields. He believed that philosophy had no mystique, it was one of the sciences. A good philosophy of biology, he might say, was a good biologist with an interest in methodological questions. He was also a very capable philosopher of science, questioning simplistic assumptions of his predecessors and proposing more realistic views of scientific language and its problems.

I have a great deal of respect for his technical contributions, a great deal of sympathy for his more philosophical proposals, and agree that most of the problems he poses are genuine. But sometimes he's just plain wrong. For instance, Quine never really dealt with the problem of induction. He noted we make proposals that get around it, but never had anything positive to say about how we do or how we should. The standard story since Bacon, is to start with a confused concept (such as heat), then apply logical analysis and observation (really, pepper heat isn't the same thing as flame really), and then find experiments that will help exclude more pseudo-content ("We must make a more diligent inquiry into this instance; for herbs and green and moist vegetables appear to possess a latent heat so small, however, as not to be perceived by the touch in single specimens, but when they are united and confined, so that their spirit cannot exhale into the air, and they rather warm each other, their heat is at once manifested, and even flame occasionally in suitable substances."). This story is older than Bacon, Aristotle and Plato would find much familiar with it. Plato would have avoided observation, both slighted experiment. This process will supposedly, by an evolutionary process, bring clarity to your language, at least as far as possible. Plato would have said clarity to one's soul!

This story was further complicated by Hume, who realized that a state of certainty and complete was not attainable for certain important concepts. For instance, to know if a truly was a cause of b, we'd have to wait for all time to see if their constant conjugation was truly constant - and even then we'd have to worry about confounding! Hume realized that what we had - and what - was an imperfect and probabilistic knowledge. Hume's realization leads to a great deal of interesting and practical philosophy, as mentioned before.

A monkey wrench was thrown by a modern philosopher, Nelson Goodman. Goodman introduced the concepts of "grue" and "bleen". In our usual language, grue might mean "green before judgement day, blue thereafter" and bleen might mean "blue before judgement day, green thereafter". Of course, in a grue/bleen language, green would mean "grue before judgement day, bleen thereafter". This is no artificial philosophical problem. When YouTube, Netflix or Amazon offers you something you'd never want in a million years, you are running into a Goodman Problem. They divide books into categories, then try to learn from your history to divide the categories you like from those you don't like. This problem is endemic in the sciences. Take economics. Imagine a monetary theorist who wants to learn about money. Money is a complicated concept. What monetary aggregate (aggragates?) might Anna Schwartz want to look at? The problem could easily be a grue problem ("M1 is important before the legal situation changed, after you want to look at M2", etc.).

What did Quine give us? Advice? Strictures? Criticism?

No. Instead Quine chose to attack the reasonable ideas of centuries of philosophers and methodologists. He links several problems together, which is a nice thing for a philosopher to do. But does he give anything to the practicing scientist? No, instead he proposes a mystical quality called "natural kinds" - which is less helpful even than the original analysis of Goodman (Goodman said that grue was not "projectable", you couldn't learn from it, which is overstated - as the comparison to monetary theory makes clear - but at least sits the problem where it is). Quine says that people make inductions on natural concepts that are usually projectable, which is only barely true, fairly false, and extremely misleading. People don't necessarily "induct on natural concepts", as made clear in - for instance - Probably Approximately Correct by Leslie Valiant. They don't take those concepts as static and given, they'd be fine if they learned some of the things they dealt with are grue-like - not that they could do anything about it if they were! They should worry if they find out that some of the things they take for granted are more sensitive and state dependent than it seems. There is a cliche in linkbait journalism "Why Things Are Worse Than They Seem". The question isn't, "What's wrong with grue?" it's "Given that grues are so common, why do we induct so well?". This is a difficulty for Bayesian theory, but I think that Leslie Valiant goes over why it isn't for other theories of representing (partial, probabilistic) knowledge and (Hume-friendly) induction.

Quine's difficulty is with his tools. He is a logician, used to a static, perfect language. When he did metaphysics, he noted ordinary reality is likely isomorphic to some four-dimensional manifold. He didn't worry about such Hegelian mysteries as languages and truth functions that are themselves dynamic and evolving. This blinded him and led him to fail to innovate, or even understand learning theory and related areas. In addition, he did not appreciate the possibility of theory-light inductive techniques of the kind seen in learning theory.

Natural Kinds are not a natural kind, but neither is anything else. The real problem is that they are misleading and useless, and get in the way of how these problems are really solved.