The logic of questions and answers, also known as erotetic logic, can be approached in different ways. The most general approach treats it as a branch of epistemic logic. The connection is mediated by what are known as the “desiderata” of questions. Given a direct question—for example, “Who murdered Dick?”—its desideratum is a specification of the epistemic state that the questioner is supposed to bring about. The desideratum is an epistemic statement that can be studied by means of epistemic logic. In the example at hand, the desideratum is “I know who murdered Dick,” the logical form of which is K_I(∃x/K_I) M(x,d). It is clear that most of the logical characteristics of questions are determined by their desiderata.

In general, one can form the desideratum of a question from any “I know that” statement—i.e., any statement of the form K_IA, where A is a first-order sentence without connectives other than conjunction, disjunction, and negation that immediately precedes atomic formulas and identities. The desideratum of a propositional question can be obtained by replacing an occurrence of the disjunction symbol ∨ in A by (∨/K_I). The desideratum of a wh-question can be obtained by replacing an existential quantifier (∃x) by (∃x/K). Desiderata of multiple questions are obtained by performing several such replacements in A.

The opposite operation consists of omitting all independence indicator slashes from the desideratum. It has a simple interpretation: it is equivalent to forming the presupposition of the question. For example, suppose that this is done in the desideratum of the question “Who murdered Dick?”—viz., in “I know who murdered Dick,” or symbolically K_I(∃x/K_I) M(x,d). Then the result is K_I(∃x) M(x,d), which says, “I know that someone murdered Dick,” which is the relevant presupposition. If it is not satisfied, no answer will be forthcoming to the who-question.

The most important problem in the logic of questions and answers concerns their relationship. When is a response to a question a genuine, or “conclusive,” answer? Here epistemic logic comes into play in an important way. Suppose that one asks the question whose desideratum is K_I(∃x/K_I) M(x,d)—that is, the question “Who murdered Dick?”—and receives a response “P.” Upon receiving this message, one can truly say, “I know that P murdered Dick”—in short, K_IM(P,d). But because existential generalization is not valid in epistemic logic, it cannot be concluded that K_I(∃x/K_I) M(x,d)—i.e., “I know who murdered Dick.” This requires the help of the collateral premise K_I(∃x/K_I) (P=x). In other words, one will have to know who P is in order for the desideratum to be true. This requirement is the defining condition on conclusive answers to the question.

This condition on conclusive answers can be generalized to other questions. If the answer is a singular term P, then the “answerhood” condition is K_I(∃x/K_I) (P=x). If the logical type of an answer is a one-place function F, then the “conclusiveness” condition is K_I(∀x)(∃y/K_I)(F(x)=y). Interpretationally, this condition says, “I know which function F is.”

The need to satisfy the conclusiveness condition means that answering a question has two components. In order to answer the experimental question “How does the variable y depend on the variable x?” it does not suffice only to know the function F that expresses the dependence “in extension”—that is to say, only to know which value of y = F(x) corresponds to each value of x. This kind of information is produced by the experimental apparatus. In order to satisfy the conclusiveness condition, the questioner must also know, or be made to know, what the function F is, mathematically speaking. This kind of knowledge is mathematical, not empirical. Such mathematical knowledge is accordingly needed to answer normal experimental questions.

On the basis of a logic of questions and answers, it is possible to develop a theory of knowledge seeking by questioning. In the section on strategies of reasoning above, it was indicated how such a theory can serve as a framework for evaluating ampliative reasoning.