The problem: if a scientist abandons theory A after deciding that it does not stand up to criticism (say, the theory fails a crucial experiment), the scientist could make the wrong choice. Theory A could very well be true, or be more approximately true (have more verisimilitude), than the replacement theory B.
Why is this a problem?
- The crucial experiment could produce a false positive, so that a scientist rejects the theory rather than rejecting the result of the test. Naturally, the scientist’s replacement theory B would have less verisimilitude.
- The crucial experiment produces a true outcome, but theory B is more approximately true than theory A over this small range; however, theory B has overall less verisimilitude than theory A.
This problem applies to any number of crucial experiments: a scientist may abandon a theory with a high degree of objective verisimilitude because he mistakenly thinks it has a low degree of verisimilitude.
The set of preformed crucial experiments will be very small, smaller than all crucial experiments available to the scientist at any one time, which in turn will be very, very small compared to all crucial experiments. This further assumes that the results of the tests are easily decidable.
Think of it this way: the scientist has insufficient reasons, which amount to nothing, after preforming a crucial experiment. Now, just keep adding additional insufficient reasons. What does the scientist have? Nothing.
What have we learned? There cannot be any evidence that anything can raise the objective probability of future success.
Assume that 1 is not an immediate problem. All results of tests are conclusive. The problem still remains, and appears to be far more robust and serious for the scientist than Quine’s problem. Now, how do we deal with 2?
One solution is to tentatively reject A and adopt B. After all, they’re only theories. Truth takes second to coherence, but the rule of operations is the negation of Quine’s holism and goes against Popper’s claim that scientists are interested in increasing verisimilitude in scientific theories. If anything, this best approximates van Fraassen’s position on empirical adequacy.