In contrast to fundamental physics, consider the project being pursued by one of the special sciences—say, molecular biology. Molecular biologists are certainly not interested in identifying patterns that hold everywhere in the universe; biologists have relatively little to say about what happens inside the sun (except perhaps to note that the conditions would make it difficult for life to prosper there). They are, instead, concerned with the behavior of a relatively small sub-set of regions of the universe. So far, the patterns they’ve identified have been observed to hold only on some parts of Earth, and that only in the last few billion years. It’s clearly no criticism of molecular biology to point out that it has nothing to say on the subject of what happens inside a black hole—that kind of system is (by design) outside molecular biology’s domain of interest. Just as in the case of S1-2 above, this restriction of domain lets molecular biologists focus their efforts on identifying patterns that, while they aren’t universal, facilitate predictions about how a very large class of physical systems behave.
What exactly is the domain of inquiry with which molecular biology is concerned? That is, how do molecular biologists carve up the world so that the patterns they identify hold of systems included in that carving? It is rather unusual (to put it mildly) for the creation of a domain in this sense to be a rapid, deliberate act on the part of working scientists. It is unusual, that is, for a group of people to sit down around a table (metaphorical or otherwise), pick out a heretofore
fundamental in the sense that it stands in an asymmetric relationship to the rest of science: generalizations of the special sciences are not allowed to contradict the generalizations of fundamental physics, but the reverse is not true; if the fundamental physicists and the biologists disagree, it is the biologist who likely has done something wrong. They call this the “Primacy of Physics Constraint” (PPC). It seems to me that while this is certainly true—that is, that it’s certainly right that the PPC is a background assumption in the scientific project—the way I’ve put the point here makes it clear why the PPC holds.
- It’s worth noting, though, that the search for habitable planets outside our own solar system is guided by the patterns identified by biologists studying certain systems here on Earth. This is an excellent case of an application of the kind of projectability we discussed above: biologists try to predict what planets are likely to support systems that are relevantly similar to the systems they study on Earth based on patterns they’ve identified in those terrestrial systems. It remains to be seen whether or not this project will prove fruitful.