There are two distinct types of coherentism. One refers to the coherence theory of truth. The other is belief in the coherence theory of justification — an epistemological theory opposing foundationalism and offering a solution to the regress argument. In this epistemological capacity, it is a theory about how belief can be justified.
As a theory of truth coherentism restricts true sentences to those that cohere with some specified set of sentences. Someone's belief is true if and only if it is coherent with all or most of his or her other beliefs. Usually, coherence is taken to imply something stronger than mere consistency. Statements that are comprehensive and meet the requirements of Occam's razor are usually to be preferred.
As an illustration of the principle, if people lived in a virtual reality universe, they could see birds in the trees that aren't really there. Not only are the birds not really there, but the trees aren't really there either. The people know that the bird and the tree are there, because it coheres with the rest of their experiences in the virtual reality. Talking about coherence is an abstract way of talking about the things that the people really know, without regard for whether they are in a virtual reality or not.
Perhaps the best-known objection to a coherence theory of truth is Bertrand Russell's. Russell maintained that since both a belief and its negation will, individually, cohere with at least one set of beliefs, this means that contradictory beliefs can be shown to be true according to coherence theory, and therefore that the theory cannot work. However, what most coherence theorists are concerned with is not all possible beliefs, but the set of beliefs that people actually hold. The main problem for a coherence theory of truth, then, is how to specify just this particular set, given that the truth of which beliefs are actually held can only be determined by means of coherence.
Both coherence and foundationalist theories of justification attempt to answer the regress argument, a fundamental problem in epistemology that goes as follows. Given some statement P, it appears reasonable to ask for a justification for P. If that justification takes the form of another statement, P', one can again reasonably ask for a justification for P', and so forth. There are three possible outcomes to this questioning process:
An infinite series appears to offer little help, since it is basically impossible to check that each justification is satisfactory. Relying on such a series quickly leads to skepticism.
A loop begs the question. Coherentism is sometimes characterised as accepting that the series forms a loop, but although this would produce a form of Coherentism, this is not what is generally meant by the term.
One might conclude that there must be some statements that, for some reason, do not need justification. This view is called foundationalism. For instance, rationalists such as Descartes and Spinoza developed axiomatic systems that relied on statements that were taken to be self-evident: 'I think therefore I am' is the most famous example. Similarly, empiricists take observations as providing the foundation for the series.
Foundationalism relies on the claim that it is not necessary to ask for justification of certain propositions, or that they are self-justifying. If someone makes an observational statement, such as 'it is raining', it does seem reasonable to ask how they know - did they look out the window? Did someone else tell them? Did they just come in shaking their umbrella? Coherentism insists that it is always reasonable to ask for a justification for any statement. Coherentism contends that foundationalism provides an arbitrary spot to stop asking for justification and so that it does not provide reasons to think that certain beliefs do not need justification.
Coherentism denies the soundness of the regression argument. The regression argument makes the assumption that the justification for a proposition takes the form of another proposition: P" justifies P', which in turn justifies P. For coherentism, justification is a holistic process. P is not justified as a part of some inferential chain of reasoning, but because it coheres with some system of which it forms a part. Usually the system is taken to be the complete set of beliefs of the individual or group, that is, their theory of the world.
It is necessary for coherentism to explain in some detail what it means for a system to be coherent. At the least, coherence must include logical consistency. It also usually requires some degree of integration of the various components of the system. A system that contains more than one unrelated explanation of the same phenomenon is not as coherent as one that uses only one explanation, all other things being equal. Conversely, a theory that explains divergent phenomena using unrelated explanations is not as coherent as one that uses only one explanation for those divergent phenomena. These requirements are variations on Occam's razor. The same points can be made more formally using Bayesian statistics. Finally, the greater the number of phenomena explained by the system, the greater its coherence.
The main criticism facing coherentism, the isolation objection, is probably simplest to state from the point of view of someone who holds to the correspondence theory of truth. This states that there is no obvious way in which a coherent system relates to anything that might exist outside of it. So, it may be possible to construct a coherent theory of the world, which does not correspond to what actually occurs in the world. In other words, it appears to be entirely possible to develop a system that is entirely coherent and yet entirely untrue.
It is surprisingly difficult to even state the problem from the point of view of a coherentist, because the phrase correspond to reality has a different meaning in a coherentist system. For a coherentist, reality is exactly the entire coherent system. It is simply not possible for a coherent theory not to correspond to reality, if reality is the very same thing as the entire coherent system.
Put another way, coherentists might reply to the critic that any substantial system that was not true would by definition contain some contradictions, and so be incoherent.
This should become clear by looking at the differences between a coherentist and correspondence account of a scientific advance. Newtonian mechanics was shown to be inconsistent with certain experiments, notably the Michelson-Morley experiment. The theory used by physicists was thereafter changed from Newtonian to relativistic mechanics.
One who held to a correspondence theory might say that there was an apparent lack of correspondence between the model (physics) and reality, and that the model was altered in order that it correspond to the observed facts.
A coherentist account might claim that before the Michelson-Morley experiment, Physics formed a coherent theory. But then the experiment was performed. These experimental results form a part of the account, yet the results were inconsistent with the expectations of the accepted theory. Thus the account was shown to be less coherent. This inconsistency was resolved by the development of Relativistic mechanics.
Any lack of correspondence of the theory with reality may eventually lead to a lack of coherence within the theory, and this leads to a modification of the theory to restore its coherence. There would be little or no practical difference between a Coherentist account and a Correspondence account of theory change.
Another problem coherentism has to face is the Plurality objection. There is nothing within the definition of coherence which makes it impossible for two entirely different sets of beliefs to be internally coherent. Thus there might be several such sets. But if one supposes that there can only be one complete set of truths, coherentism must provide a way to choose between these competing sets.
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