Van Fraassen thinks that when we accept a scientific theory, we should be bracket the theory’s claims about unobservable entities, but believe everything else.
An oddity has occurred to me. Suppose a theory talks about certain microorganisms that are just under the minimum size for human visual observation. But when you have two things that are just under the minimum observable size side-by-side, the pair is observable. So, oddly, we will believe in pairs without believing in individuals.
For further oddity, now imagine that Alpha and Beta are such a side-by-side pair. Then we believe in Alphabeta, the pair of Alpha and Beta. Suppose Alpha swims a little away from Beta. Now, Alphabeta disappears from view. But Alphabeta is still observable. To observe Alphabeta, all we need to do is to coax Alpha and Beta to swim to each other. So on van Fraassen grounds, we should still continue to believe in Alphabeta even when temporarily we cannot see it due to the separation of Alpha from Beta. (Compare: a very thin sheet is still observable when it is edge-on, even though it can only be seen when tilted to the line of sight.)
But it is absurd to believe in a pair of organisms, at different ends of a test-tube, without believing in either organism.
One way out for van Fraassen is to adopt a sparse ontology on which there are no pairs. But while I like such an ontology, I don’t think van Fraassen will want to do that, as he wants to believe in observable objects that science talks about, such as planets.
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