What Is an Explanandum?
An explanandum is the phenomenon, pattern, event, relation, or outcome that an explanation is intended to explain.
In simpler terms:
The explanandum is what is to be explained.
The term is useful because scientific explanation cannot be assessed adequately without first identifying the explanatory question.
A scientific account may contain accurate observations, successful predictions, detailed mechanisms, mathematical relations, or well-supported causal claims while still failing to explain the phenomenon actually at issue. What matters is not only whether the information is scientifically sound, but whether it bears appropriately on the explanandum.
Explanandum and Explanatory Target
Explanandum and explanatory target are closely related, but they need not always be interchangeable.
The explanandum identifies what is to be explained. The explanatory target can specify more precisely the aspect, contrast, dependency, spatial or temporal grain, or explanatory question at issue.
For example, the same biological phenomenon might support questions about:
- how it is produced;
- why it occurs under particular conditions;
- why one outcome occurs rather than another;
- what mechanism maintains it;
- what mathematical relation constrains it;
- or how it arose historically.
The phenomenon may be the same while the explanatory targets differ.
This is why specifying the explanandum is necessary but may not by itself completely specify the explanatory task.
Why the Explanandum Matters
Explanatory adequacy is target-sensitive.
An explanation can therefore be highly successful for one explanandum or explanatory target while being irrelevant to another.
This does not mean that explanatory adequacy is arbitrary. It means that an explanation must be evaluated in relation to the question it claims to answer, together with the evidence and explanatory relations relevant to that question.
Identifying the explanandum helps prevent several common errors, including:
- comparing explanations that address different questions;
- treating additional detail as automatically providing better explanation;
- assuming predictive success answers every explanatory question;
- treating a mechanism as relevant merely because it concerns the same system;
- and interpreting different explanatory products as competitors when their targets differ.
Explanandum and Explanans
The traditional distinction is between the explanandum and the explanans.
- Explanandum — what is to be explained.
- Explanans — what is offered as the explanatory answer.
The distinction does not determine what form a successful explanation must take. Depending on the explanatory problem, the relevant answer may involve causal dependencies, mechanisms, mathematical or structural relations, contrasts, historical processes, or other scientifically warranted relations.
The distinction instead clarifies the basic architecture of an explanatory claim:
What requires explanation, and what is being offered to explain it?
Key Point
The explanandum is what is to be explained. Identifying it is a necessary first step in determining whether an explanation actually addresses the scientific question at issue.