Underdetermination (sometimes indeterminacy of data to theory) is a term used in the discussion of theories and their relation to the evidence that is cited to support them. Arguments from underdetermination are used to support epistemic relativism by claiming that there is no good way to certify a theory based on any set of evidence. A theory (or statement or belief) is underdetermined if, given the available evidence, there is a rival theory which is inconsistent with the theory that is at least as consistent with the evidence. Underdetermination is an epistemological issue about the relation of evidence to conclusions.

History of underdetermination

Underdetermination receives its first modern treatment in the work of René Descartes. Among other skeptical arguments, Descartes presents two arguments demonstrating underdetermination.

Descartes's dream argument points out that, while dreaming, perceived experiences (for example, falling) do not necessarily contain sufficient information to deduce the true situation (being in bed). Since one cannot always distinguish dreams from reality, one cannot rule out the theory that one is presently dreaming rather than having veridical experiences; thus the theory that one is having a veridical experience is underdetermined.

Descartes's demon argument is a variant of the dream argument that posits that all of one's experiences and thoughts might be manipulated by a very powerful being (an "evil demon") that always deceives. Once again, so long as the perceived reality appears internally consistent to the limits of one's limited ability to tell, the situation is indistinguishable from reality, one cannot logically determine between correct belief from being misled; this is another version of underdetermination.

David Hume is the source of another important underdetermination argument, the problem of induction.

Underdetermination claimed propriety in the twentieth century with the famous work of Thomas S. Kuhn, a theoretical physicist turned philosopher. His famous work, The Structure of Scientific Revolutions offered an alternative to linear models of scientific progress. Besides presenting a general epistemological problem, underdetermination has been used to argue against theories in the philosophy of science, and especially against scientific realism.

Types of underdetermination

Underdetermination can be divided into weak and strong underdetermination. To claim that a theory is weakly underdetermined is to say that the currently available evidence fails to prove it, but some evidence collected in the future might conceivably be able to. To claim that a theory is strongly underdetermined is to claim that it is fundamentally impossible to acquire evidence that could completely settle the dispute between the rival theories.

Another distinction is between deductive and inductive underdetermination. For two theories to be deductively underdetermined means that the available evidence does not completely contradict either theory. The case of inductively underdetermined theories is more problematic; not only are the theories compatible with the available evidence, but even attempts to determine which theory is better fail.

The possible combinations of these two distinctions yield four types of underdetermination, though by definition a case of weak inductive underdetermination admits the possibility of its own resolution into mere deductive underdetermination through the acquisition of future evidence, otherwise it would fall under strong underdetermination. In turn, it could be argued that it is impossible for a theory to be truly strongly, inductively underdetermined: it would have to admit absolutely no possibility of valid evidence in the favor of other theories or against itself, which would necessitate it not making any novel predictions, which in turn is a valid inductive argument against it (see Occam's razor).

Claims of weak underdetermination tend to center on what evidence happens to be available for some specific set of theories, while strong underdetermination often involves more general epistemological claims about what kind of evidence is possible or viable at all, either for a particular theory or for theories in general. It is widely accepted, but not universally, that all theories are weakly underdetermined; and that all theories are strongly underdetermined for most practical purposes.

Support for underdetermination

To show that a theory is underdetermined, one must show that there is a rival theory, equally well supported by the standards of evidence. A trivial example of underdetermination is the addition of an observer. For example, there is the theory that "objects near earth fall toward it when dropped". A rival theory is that "objects near earth fall when dropped but only if we check to see that they do". This rival is generated by taking any accepted theory and appending to it "whenever we look for evidence." Since one may append this to any theory, all theories are at least trivially underdetermined. If one considers such modifications of theories to be illegitimate then such "tricks" are not to be considered demonstrations of underdetermination.

More serious cases of underdetermination are illustrated where a theory admits several possibilities between which the evidence for the theory says nothing. Isaac Newton's mechanics provides such an example. According to Newton, there is an absolute space in which events are located but all we can ever detect are differences between velocities. Hence, it is equally consistent with Newton's theory to say that the solar system is at rest as to say that it moves at a velocity of 37 m/s in the direction from the center of the earth to the north pole. Newton himself says these two possibilities are indistinguishable.

Arguments involving underdetermination

Arguments involving underdetermination attempt to show that there is no reason for belief regarding some theory because it is underdetermined by the evidence. Since the evidence does not show that the theory is the uniquely true hypothesis, there is no reason to believe it rather than some equally supported rival.

Because arguments involving underdetermination involve both a claim about what the evidence is and that such evidence underdetermines a theory, it is often useful to separate these two claims within the underdetermination argument as follows:

  1. All the evidence of a certain type underdetermines which of several rival theories is correct.
  2. Only evidence of that type is relevant to believing one of these theories.
  3. Therefore, there is no evidence for believing one among the rival theories.

The first premise makes the claim that a theory is underdetermined. The second says that rational decision (i.e. using available evidence) depends upon evidence that underdetermines the theory.

Underdetermination and general skeptical arguments

Some of the most powerful skeptical arguments appeal to the fact that all the evidence we could ever gather would still fail to determine which theory was true. It would remain compatible with 'skeptical hypotheses' like the maintenance of a complex illusion by Descartes' evil demon or (in a modern updating) the machines who run the Matrix. The skeptic argues that this undermines any claims to knowledge, or even (by internalist definitions), justification.

Philosophers have found this argument very powerful. Hume felt it was unanswerable, but observed that it was in practice impossible to accept its conclusions. Influenced by this, Kant held that while the nature of the 'noumenal' world was indeed unknowable, we could aspire to knowledge of the 'phenomenal' world. A similar response has been advocated by modern anti-realists.

It should be noted that Underdetermination is also a type of Middle ground fallacy: it presupposes, because there is conflict between different views that claim to be correct, that none of them actually are correct.

Underdetermination and philosophy of science

In the philosophy of science, underdetermination is often presented as a problem for scientific realism, which holds that we have reason to believe in unobservable entities (such as electrons) talked about by scientific theories. One such argument proceeds as follows:

  1. All the observational evidence for the unobservable entities of scientific theories underdetermines the claims of the theory about unobservable entities.
  2. Only the observational evidence is relevant to believing a scientific theory.
  3. Therefore, there is no evidence for believing what scientific theories say about unobservable entities.

Particular responses to this argument attack both the first and the second premise (1 and 2). It is argued against the first premise that the underdetermination must be strong and/or inductive. It is argued against that second premise that there is evidence for a theory's truth besides observations; for example, it is argued that simplicity, explanatory power or some other feature of a theory is evidence for it over its rivals.

A more general response from the scientific realist is to argue that underdetermination is no special problem for science, because, as indicated earlier in this article, all knowledge that is indirectly supported by evidence suffers from it - for example, conjectures concerning unobserved observables. It is therefore, ironically, too powerful an argument to have any significance in the philosophy of science, since it does not cast doubt uniquely on conjectured unobservables.

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