 First, consider the Argument from Underdetermination. This argument starts by pointing out that for any theory, there are rival theories that are empirically equivalent to it—the theories make all the same predictions about what’s observable, but differ only with regard to what’s unobservable. The argument goes on to say that it follows that all the empirically equivalent theories are equally believable, and hence belief in the truth of any one of those empirically equivalent theories must be irrational.