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Underdetermination January 26, 2009

Posted by ebrister in Nuts and Bolts.
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We read an excerpt from Duhem’s The Aim and Structure of Physical Theory (1906) which introduced the notion of underdetermination of theory by evidence. The underdetermination thesis holds that individual statements cannot be empirically confirmed or disconfirmed by a single observation. This thesis upholds holism: that our theories are composed of collections of statements which are evaluated as a whole. Disconfirming evidence can be dealt with by making any number of changes to this collection of statements.

Duhem writes:

In sum, the physicist can never subject an isolated hypothesis to experimental test, but only a whole group of hypotheses; when the experiment is in disagreement with his predictions, what he learns is that at least one of the hypotheses constituting this group is unacceptable and ought to be modified; but the experiment does not designate which one should be changed.

This challenges two common beliefs about how science works.

1. We cannot give a complete logical reconstruction of scientific reasoning which makes a conclusion seem to have been compelled by evidence. This is obviously the case for induction from evidence because it’s possible to generalize from a limited set of observations in any number of ways. But it’s also true for deduction; say, along Popper’s lines. Due to underdetermination, we can’t make a conjecture, deduce the results we ought to see, and then know for sure which part of the theory has been falsified when the deduced prediction is disconfirmed.

2. If we have two theories that are empirically equivalent except for giving different predictions as to the result of a crucial experiment, then running that experiment ought to give us the ability to choose between them. However, the problem is that this assumes that there is no overlooked alternative theory–that the two we are choosing between are the only two possible explanatory theories. This is why Duhem says:

Unlike the reduction to absurdity employed by geometers, experimental contradiction does not have the power to transform a physical hypothesis into an indisputable truth; in order to confer this power on it, it would be necessary to enumerate completely the various hypotheses;…but the physicist is never sure to have exhausted all the imaginable assumptions.


© Kenneth Spring, Microscopy Resource Center

The class’s opinion last week seemed to be that understanding underdetermination was straightforward. I would like to point out, though, that the implications of the thesis are far-reaching and still of contemporary interest. For instance, here is the program for a conference this March on underdetermination.

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