UNDERDETERMINATION
Underdetermination is an epistemological issue about the relation of evidence to conclusions. A theory (or statement or belief) is underdetermined if, given the evidence for the theory, there is a rival theory inconsistent with that theory but as consistent with all the evidence for that theory. That a theory is underdetermined is a premise upon which skepticism often rests. Since skepticism is a view that challenges beliefs about knowing and people value knowledge, people often speak of "the underdetermination problem."
History of underdetermination
Skepticism, and consequently underdetermination, has a history at least as old as the ancient Greeks. This issue has continued to be one in philosophy ever since.
Underdetermination receives its most famous treatment in the work of Rene Descartes, who attempted to argue from a skeptical position to a position in which he had significant knowledge. Among other skeptical arguments, Descartes presents two that rest on a premise that invokes underdetermination. (He rejects all such arguments but they have been very influential in epistemology.) His dream argument points out that while dreaming one often feels as if awake and hence cannot discern the failure of one's experiences (as if, for example, falling) to represent reality (being in bed). Since one cannot distinguish dreams from reality, one cannot rule out the theory that one is presently dreaming rather than having veridical experiences; this, then, is a claim that the theory that one is having a veridical experience is underdetermined. Descartes's demon argument points out that all of one's experiences and thoughts might be manipulated by a very powerful being (an "evil demon") that always deceives. Once again, since one cannot rule out that one is deceived by such a powerful being, one cannot rule out that one believes correctly rather than being misled; this is another version of underdetermination.
David Hume is another source of an important underdetermination argument, the problem of induction.
In the twentieth century, 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
There is a subtle distinction between two types of underdetermination, weak and strong. To claim that theory is weakly underdetermined by the data is to say that the currently available evidence fails to prove it, but in theory, 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 some piece of evidence that would completely settle the dispute between the two (or more) rival theories in question.
Another distinction is between deductive and inductive underdetermination. For theories to be deductively underdetermined means that all of the evidence which is taken into consideration is equally consistent with both, i.e. for both of them to be at least possible in theory. The case of inductively underdetermined theories is even more problematic; not only are the theories consistent with the evidence in question, but even attempts to determine which of them is better supported by induction yield a result where neither has the advantage.
The possible combinations of these two distinctions yield four types of underdetermination altogether, though in truth by definition a case of weak inductive underdetermination admits the possibility of its own resolution into merely deductive by the acquiring of future evidence, or 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, for a particular theory or for theories in general. It is widely (though perhaps not completely) accepted that all theories are weakly underdetermined; though not without controversy, it is also widely accepted that all theories are strongly underdetermined, if not in principle then for most practical purposes. Even a set of beliefs that has been shown to be false by new evidence can adapt via ad hoc hypotheses, retracting the hypotheses that have been contradicted and asserting new ones that are consistent with the new evidence in their stead, while retaining the core content of the theory. In addition to claiming that for a particular theory there is at least one rival theory consistent with all evidence for the theory, it is often claimed that there are infinitely many such theories.
Support for underdetermination
To show that a theory is underdetermined, one must show that there is a rival theory just as good as some other by the standards of evidence. A trivial but instructive example involves objects falling to earth. A natural theory is that all objects near earth fall toward it when dropped. A rival is that objects near earth fall when dropped and we check to see that they do. The rival is easily generated by taking an accepted theory and appending to it "whenever we look for evidence." Since one may append this to any theory, it is easily shown that a theory is at least trivially underdetermined. Many people have thought that such modifications of theories are illegitimate and such "tricks" are often found objectionable demonstrations of underdetermination.
More serious cases of underdetermination are illustrated by showing that a theory itself 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:
- All the evidence of a certain type underdetermines which of several rival theories is correct.
- Only evidence of that type is relevant to believing one of these theories.
- 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.
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:
- All the observational evidence for the unobservable entities of scientific theories underdetermines the claims of the theory about unobservable entities.
- Only the observational evidence is relevant to believing a scientific theory.
- 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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