Inductive realism and approximate truth

Theory Status, Inductive Realism, and Approximate Truth: No Miracles, No Charades

This paper offers two interesting things: a way of looking at scientific practice, and a case study of the eradication of smallpox. The paper also promises an explication of the concept of approximate truth, though I'm sceptical about whether it really delivers on this point.

Hunt starts by outlining what he calls the "inductive realist" model of scientific practice. This is encapuslated by a diagram (p.164) that distinguishes "theory proposals", "theory status", "theory use" and "external world". He then explains the various interconnections between these things. As a broad strokes outline of scientific practice, this seems to fit with what I understand of the process. As a tool for clarifying the concept of approximate truth, it falls somewhat short. What Hunt's analysis of approximate truth boils down to is the claim that

accepting a theory ... as approximately true is warranted when the evidence related to the theory is sufficient to give reason to believe that something like the specific entities ... posited by the theory is likely to exist in the world external to the theory. (p. 169, italics in original)

Now, stripping this claim of its numerous caveats, and disclaimers it becomes: "A theory is approximately true when there is something like the theoretical entities in the world." Without a further explanation of what it means for a theoretical entity to be "like" a physical entity, this is no explanation at all.

Hunt also seems to conflate metaphysical notions of "truth"and epistemic considerations for the credibility of some proposition. If this is supposed to be a contribution to the scientific realism debate, then it seems like there has to be something ontological at stake. But as the above quoted passage shows, Hunt is always couching his claims about truth in terms of what one has "reason to believe"...

After this somewhat unsatisfactory analysis of approximate truth, Hunt turns to his smallpox case study. It was common knowledge in Europe in the 18th century that if you'd survived smallpox, you were no longer susceptible to catching it. It was also discovered around this time that those inoculated by being exposed to a small amount of the virus had a smaller risk of death than those who caught the disease in the usual way. The death rate for inoculation was still something like 1%, however, compared to 12% for the normal form of the disease.

Edward Jenner followed up on stories that people who had contracted the milder disease "cowpox" were also immune to smallpox. He wondered whether inoculating people with cowpox would give them an immunity to smallpox. His early trials were successful, and within a few years smallpox vaccines were available. By 1900 smallpox was all but eradicated in North America and Europe. (It took until the 80s before the disease was finally declared eradicated worldwide).

Hunt tries to use his analysis of approximate truth to explain the success of the smallpox vaccine. This seems a very strange thing to do: he attributes something called "the smallpox theory" to everyone from Jenner in the 1790s to the WHO in 1980. There was no such theory that they shared. Jenner didn't have a modern understanding of virology; indeed, it's unclear that he had any "theory" about smallpox at all. All he needed was a relatively low-level inductive move from "Smallpox inoculation causes immunity" and "Cowpox is like smallpox" to "Cowpox inoculation will cause immunity". He had no idea that smallpox and cowpox were viruses, or that inoculation/vaccination work by allowing the body to generate antibodies. Hunt lists 7 statements that together explain the success of the smallpox vaccination programme. Only the first of these statements is something that Jenner would have "known" in the 1790s.

If Jenner ever made any explicit predictions about the effectiveness of his vaccine, the accuracy of those predictions cannot be explained by Jenner's having found an approximately true "smallpox theory". We now understand Jenner's success in terms of viruses and antibodies, but Jenner didn't know about these things. What the realist typically wants to do when trying to explain some scientific success is show that the theory the scientists had at the time is approximately true with respect to our current understanding. Now in the case of the smallpox vaccine, that doesn't seem plausible: Jenner didn't really have much of a theory, so there isn't any way to demonstrate correspondence between it and current understanding. In short, Jenner wasn't successful because he had some grand theory of disease that corresponds well with the world, he was successful because he was lucky enough to stumble upon a relatively low-level inductive leap that held true: cowpox is enough like smallpox for the same inoculation procedure to work.

I'd like to finish with two aspects of the paper that I did like. First, Hunt insists that we must be fallibilist about our theories: we must not assume that science never makes mistakes. Second, despite the odd use he puts it to, the smallpox case study is an interesting one for philosophy of science: that Jenner was so successful without any sort of high level theory of disease is an interesting case.