Explanandum and explanans

An explanandum is something that needs to be explained and its explanans is the answer (or explanation) of that phenomenon. For instance, someone asks why there is smoke on the horizon, they seek an explanation for this explanandum. They want an explanans, or set of claims that will make plain the explanandum. Continuing the example, if 'There is smoke on the horizon' is an explanandum, then 'There is a fire over at the Old Mill' would be one possible explanans.

Hempel and Oppenheim (1948), in their deductive-nomological model of scientific explanation, motivated the distinction between explanans and explanandum in order to answer why-questions, rather than simply what-questions:

"the event under discussion is explained by subsuming it under general laws, i.e., by showing that it occurred in accordance with those laws, by virtue of the realization of certain specified antecedent conditions" (p.152).

Specifically, they define the concepts as follows:

"By the explanandum, we understand the sentence describing the phenomenon to be explained (not that phenomenon itself); by the explanans, the class of those sentences which are adduced to account for the phenomenon" (p.152).

The crucial comment, with respect to the scientific method, is given as follows:

"It may be said... that an explanation is not fully adequate unless its explanans, if taken account of in time, could have served as a basis for predicting the phenomenon under consideration.... It is this potential predictive force which gives scientific explanation its importance: Only to the extent that we are able to explain empirical facts can we attain the major objective of scientific research, namely not merely to record the phenomena of our experience, but to learn from them, by basing upon them theoretical generalizations which enable us to anticipate new occurrences and to control, at least to some extent, the changes in our environment" (p.154).

Other examples

• Explicandum -- that which gets explicated. Explicans -- that which gives the explication.
• Constitutum -- that which gets made up, constituted, e.g. a legal constitution. Constituens -- that which makes it up, e.g. the constituents who are authorized to make a constitution.

References

• Hempel, C.G. & Oppenheim, P. (1948). "Studies in the Logic of Explanation." Philosophy of Science, XV, pp. 135–175.