Читать книгу This is Philosophy of Science - Franz-Peter Griesmaier - Страница 45

We quite deliberately phrased the ending of Copernicus’ successful rescue of heliocentrism and the ending of Priestley’s attempted rescue of the phlogiston theory almost identically, only replacing some of the words, in order to bring out how eerily similar the two episodes are. And yet, from today’s perspective, Copernicus’ rescue appears heroic, while Priestley’s seems foolish. For one, we now know that Copernicus was right, and that Priestley was wrong. However, we need to be careful not to let hindsight play a role in evaluating their rescue attempts. From a logical perspective, they seem identical. Given their respective historical contexts, both the claim that the universe is vastly bigger than usually estimated, and that phlogiston has negative, rather than the standardly assumed positive, weight, are hopelessly ad hoc. It’s just that Copernicus got lucky and was proven right by subsequent developments, while Priestley had massively bad luck and now looks like a fool.

Could their respective fates have been predicted at the time? If Popper is right about legitimate vs. illegitimate modifications in the face of observational counterevidence, it would have to be the case that Copernicus’ modification was arguably progressive, while Priestley’s was not. Remember that a modification is progressive to the extent to which the modified theory has more content, and thus faces more potential falsifiers, than its predecessor. Copernicus’ modification postulated a very large universe. Priestley’s modification postulated a substance with negative weight. Regarding Copernicus, it is difficult to determine whether heliocentrism in a large universe has more falsifiers than heliocentrism in a small universe. Thus, it is difficult to say whether this modification was indeed progressive in Popper’s sense. The difficulty might reside in Popper’s choice of the “scientific unit” that is to be judged as progressive or not. The unit for Popper is an individual theory. His student, Hungarian-born Imre Lakatos, proposed using a wider unit, viz., a research programme. Perhaps Copernicus’ modification can be seen as progressive in virtue of being embedded in a progressive research programme, as we’ll discuss in Chapter 15 on scientific progress.

Let’s now turn to Priestley’s negative weight. There is one sense in which this proposed modification is not progressive, but rather badly ad hoc. Phlogiston was supposed to be the only substance that has negative weight. But could this hypothesis at least be falsified? In Copernicus’ case, the development of increasingly powerful telescopes provided good empirical reasons for adjusting the estimates about the size of the universe upward, until, by current estimates, we arrived at 91 billion light years. In principle, it could also have been falsified. This would seem to be impossible in the case of phlogiston. The obvious way would be to isolate phlogiston and then try to weigh it – but with what? We don’t have instruments for determining the value of negative weight. Moreover, with negative weight, would phlogiston also have to have negative mass? (Remember, weight is simply a function of mass and the gravitational constant). What would that be? Sure, the negative weight idea saved the theory from the mercury counterexample. But it would have been quite obvious that independent evidence was elusive if not impossible. In light of this, perhaps the right verdict is to say that the statement “Phlogiston has negative weight” looks quite a bit like the statement “The absolute is beautiful.” Neither one can in any clear way be falsified. Thus, by Popper’s criterion for progressive modifications, Priestley’s modification fails, as it introduces a nonfalsifiable, and thus merely protective, hypothesis.