An Impoverished Hypothetico-Deductive View of Confirmation
This is probably the most common of all objections leveled against the underdetermination thesis and it has been raised in many guises. The concern is quite straightforward and quite fundamental. Since the underdetermination thesis makes a claim about inductive or confirmatory relations, one would expect the thesis to be supported by the very long tradition of work in induction and confirmation. Yet the expositions of the underdetermination thesis seem to make only superficial contact with this literature and the arguments for the thesis—notably the three sketched above—seem to depend entirely on a flawed account of the nature of induction. That is, they invoke an impoverished version of hypothetico-deductive confirmation through which:
If two theories T and T' are each able logically to entail E, then T and T' are confirmed equally by E.
One sees immediately that this account of confirmation is troubled by an excessive permissiveness. The standard illustration of the concern is frivolous conjunction. If some theory T entails evidence E, then so does the strengthened theory T' = T & X, where X is any hypothesis, no matter how odd or frivolous. T & X then enjoys the same level of evidential support as T.
It is exactly this permissiveness that renders impoverished hypothetico-deductivism an uninteresting option in scientific practice and in the induction literature. Yet it is exactly this permissiveness that the arguments for the underdetermination thesis seek to exploit. For this impoverished version of hypothetico-deductive confirmation appears to be the relation routinely invoked in accounts of the underdetermination thesis. Empirically equivalent pairs of theories, in this literature, typically turn out to be pairs of theories that have the same observational consequences. They are automatically read as being equally confirmed by these consequences. Or we are to generate a rival theory by making compensating adjustments to the hypotheses of the original theory. The rival is equally confirmed by the common observations only if all that matters in the confirmation relation is that the two theories have identical observational consequences. In his discussion of empirical underdetermination in his "On Empirically Equivalent Systems of the World," Quine (1975, p. 313) wrote:
The hypotheses [scientists invent] are related to observation by a kind of one-way implication; namely, the events we observe are what a belief in the hypotheses would have led us to expect. These observable consequences of the hypotheses do not, conversely, imply the hypotheses. Surely there are alternative hypothetical substructures that would surface in the same observable ways.
And surely we are intended to conclude that these alternative hypothetical substructures are worthy rivals to the originals.