Within the philosophy of science there have been competing ideas about what an explanation is. Historically, explanation has been associated with causation: to explain an event or phenomenon is to identify its cause. But with the growth and development of philosophy of science in the 20th century, the concept of explanation began to receive more rigorous and specific analysis. Of particular concern were theories that posited the existence of unobservable entities and processes (atoms, fields, genes, and so forth). These posed a dilemma: on the one hand, the staunch empiricist had to reject unobservable entities as a matter of principle; on the other, theories that appealed to unobservable entities were clearly producing revolutionary results. Thus philosophers of science sought some way to characterize the obvious value of these theories without abandoning the empiricist principles deemed central to scientific rationality.
A theory of explanation might treat explanations in either a realist or an epistemic (that is, anti-realist) sense. A realist interpretation of explanation holds that the entities or processes an explanation posits actually exist–the explanation is a literal description of external reality. An epistemic interpretation, on the contrary, holds that such entities or processes do not necessarily exist in any literal sense but are simply useful for organizing human experience and the results of scientific experiments–the point of an explanation is only to facilitate the construction of a consistent empirical model, not to furnish a literal description of reality. Thus Hempel‘s epistemic theory of explanation deals only in logical form, making no mention of any actual physical connection between the phenomenon to be explained and the facts purported to explain it, whereas Salmon’s realist account emphasizes that real processes and entities are conceptually necessary for understanding exactly why an explanation works.
In contrast to these theoretical and primarily scientific approaches, some philosophers have favored a theory of explanation grounded in the way people actually perform explanation. Ordinary Language Philosophy stresses the communicative or linguistic aspect of an explanation, its utility in answering questions and furthering understanding between two individuals, while an approach based in cognitive science maintains that explaining is a purely cognitive activity and that an explanation is a certain kind of mental representation that results from or aids in this activity. It is a matter of contention within cognitive science whether explanation is properly conceived as the process and results of belief revision or as the activation of patterns within a neural network.
This article focuses on the way thinking about explanation within the philosophy of science has changed since 1950. It begins by discussing the philosophical concerns that gave rise to the first theory of explanation, the deductive-nomological model. Discussions of this theory and standard criticisms of it are followed by an examination of attempts to amend, extend or replace this first model. There is particular emphasis on the most general aspects of explanation and on the extent to which later developments reflect the priorities and presuppositions of different philosophical traditions. There are many important aspects of explanation not covered, most notably the relation between the different types of explanation such as teleological, functional, reductive, psychological, and historical explanation — that are employed in various branches of human inquiry.
Table of Contents
- Introduction
- Hempel’s Theory of Explanation
- Standard Criticisms of Hempel’s Theory of Explanation
- Contemporary Developments in the Theory of Explanation
- The Current State of the Theory of Explanation
- References and Further Reading
1. Introduction
Most people, philosophers included, think of explanation in terms of causation. Very roughly, to explain an event or phenomenon is to identify its cause. The nature of causation is one of the perennial problems of philosophy, so on the basis of this connection one might reasonably attempt to trace thinking about the nature of explanation to antiquity. (Among the ancients, for example, Aristotle’s theory of causation is plausibly regarded as a theory of explanation.) But the idea that the concept of explanation warrants independent analysis really did not begin to take hold until the 20th century. Generally, this change occurred as the result of the linguistic turn in philosophy. More specifically, it was the result of philosophers of science attempting to understand the nature of modern theoretical science.
Of particular concern were theories that posited the existence of unobservable entities and processes (for example, atoms, fields, genes, etc.). These posed a dilemma. On the one hand, the staunch empiricist had to reject unobservable entities as a matter of principle; on the other hand, theories that appealed to unobservables were clearly producing revolutionary results. A way was needed to characterize the obvious value of these theories without abandoning the empiricist principles deemed central to scientific rationality.
In this context it became common to distinguish between the literal truth of a theory and its power to explain observable phenomena. Although the distinction between truth and explanatory power is important, it is susceptible to multiple interpretations, and this remains a source of confusion even today. The problem is this: In philosophy the terms “truth” and “explanation” have both realist and epistemic interpretations. On a realist interpretation the truth and explanatory power of a theory are matters of the correspondence of language with an external reality. A theory that is both true and explanatory gives us insight into the causal structure of the world. On an epistemic interpretation, however, these terms express only the power of a theory to order our experience. A true and explanatory theory orders our experience to a greater degree than a false non-explanatory one. Hence, someone who denies that scientific theories are explanatory in the realist sense of the term may or may not be denying that they are explanatory in the epistemic sense. Conversely, someone who asserts that scientific theories are explanatory in the epistemic sense may or may not be claiming that they are explanatory in the realist sense. The failure to distinguish these senses of “explanation” can and does foster disagreements that are purely semantic in nature.
One common way of employing the distinction between truth and explanation is to say that theories that refer to unobservable entities may explain the phenomena, but they are not literally true. A second way is to say that these theories are true, but they do not really explain the phenomena. Although these statements are superficially contradictory, they can both be made in support of the same basic view of the nature of scientific theories. This, it is now easy to see, is because the terms ‘truth’ and ‘explanation’ are being used differently in each statement. In the first, ‘explanation’ is being used epistemically and ‘truth’ realistically; in the second, ‘explanation’ is being used realistically and ‘truth’ epistemically. But both statements are saying roughly the same thing, namely, that a scientific theory may be accepted as having a certain epistemic value without necessarily accepting that the unobservable entities it refers to actually exist. (This view is known as anti-realism.) One early 20th century philosopher scientist, Pierre Duhem, expressed himself according to the latter interpretation when he claimed:
A physical theory is not an explanation. It is a system of mathematical propositions, deduced from a small number of principles, which aim to represent as simply, as completely, and as exactly as possible a set of experimental laws. ([1906] 1962: p7)
Duhem claimed that:
To explain is to strip the reality of the appearances covering it like a veil, in order to see the bare reality itself. (op.cit.: p19)
Explanation was the task of metaphysics, not science. Science, according to Duhem, does not comprehend reality, but only gives order to appearance. However, the subsequent rise of analytic philosophy and, in particular, logical positivism made Duhem’s acceptance of classical metaphysics unpopular. The conviction grew that, far from being explanatory, metaphysics was meaningless insofar as it issued claims that had no implications for experience. By the time Carl Hempel (who, as a logical positivist, was still fundamentally an anti-realist about unobservable entities) articulated the first real theory of explanation (1948) the explanatory power of science could be stipulated.
To explain the phenomena in the world of our experience, to answer the question “Why?” rather than only the question “What?”, is one of the foremost objectives of all rational inquiry; and especially scientific research, in its various branches strives to go beyond a mere description of its subject matter by providing an explanation of the phenomena it investigates. (Hempel and Oppenheim 1948: p8)
For Hempel, answering the question “Why?” did not, as for Duhem, involve an appeal to a reality beyond all experience. Hempel employs the epistemic sense of explanation. For him the question “Why?” was an expression of the need to gain predictive control over our future experiences, and the value of a scientific theory was to be measured in terms of its capacity to produce this result.
2. Hempel’s Theory of Explanation
According to Hempel, an explanation is:
…an argument to the effect that the phenomenon to be explained …was to be expected in virtue of certain explanatory facts. (1965 p. 336)
Hempel claimed that there are two types of explanation, what he called ‘deductive-nomological’ (DN) and ‘inductive-statistical’ (IS) respectively.” Both IS and DN arguments have the same structure. Their premises each contain statements of two types: (1) initial conditions C, and (2) law-like generalizations L. In each, the conclusion is the event E to be explained:
C1, C2, C3,…Cn
L1, L2, L3,…Ln
————————
E
The only difference between the two is that the laws in a DN explanation are universal generalizations, whereas the laws in IS explanations have the form of statistical generalizations. An example of a DN explanation containing one initial condition and one law-like generalization is:
C. The infant’s cells have three copies of chromosome 21.
L. Any infant whose cells have three copies of chromosome 21 has Down’s Syndrome.
————————————————————————————————–
E. The infant has Down’s Syndrome.
An example of an IS explanation is:
C. The man’s brain was deprived of oxygen for five continuous minutes.
L. Almost anyone whose brain is deprived of oxygen for five continuous minutes will sustain brain damage.
—————————————————————————————————
E. The man has brain damage.
For Hempel, DN explanations were always to be preferred to IS explanations. There were two reasons for this.
First, the deductive relationship between premises and conclusion maximized the predictive value of the explanation. Hempel accepted IS arguments as explanatory just to the extent that they approximated DN explanations by conferring a high probability on the event to be explained.
Second, Hempel understood the concept of explanation as something that should be understood fundamentally in terms of logical form. True premises are, of course, essential to something being a good DN explanation, but to qualify as a DN explanation (what he sometimes called a potential DN explanation) an argument need only exhibit the deductive-nomological structure. (This requirement placed Hempel squarely within the logical positivist tradition, which was committed to analyzing all of the epistemically significant concepts of science in logical terms.) There is, however, no corresponding concept of a potential IS explanation. Unlike DN explanations, the inductive character of IS explanations means that the relation between premises and conclusion can always be undermined by the addition of new information. (For example, the probability of brain damage, given that a man is deprived of oxygen for 7 minutes, is lowered somewhat by the information that the man spent this time at the bottom of a very cold lake.) Consequently, it is always possible that a proposed IS explanation, even if the premises are true, would fail to predict the fact in question, and thus have no explanatory significance for the case at hand.
3. Standard Criticisms of Hempel’s Theory of Explanation
Hempel’s dissatisfaction with statistical explanation was at odds
