The application of logical techniques to the study of knowledge or knowledge claims is called epistemic logic. The field encompasses epistemological concepts such as knowledge, belief, memory, information, and perception. It also turns out that a logic of questions and answers, sometimes called “erotetic” logic (after the ancient Greek term meaning “question”), can be developed as a branch of epistemic logic.

Epistemic logic was developed in earnest when logicians began to notice that the use of knowledge and related concepts seemed to conform to certain logical laws. For example, if one knows that A and B, one knows that A and one knows that B. Although a few such elementary observations had been made as early as the Middle Ages, it was not until the 20th century that the idea of integrating them into a system of epistemic logic was first put forward. The Finnish philosopher G.H. von Wright is generally recognized as the founder of this field.

The interpretational basis of epistemic logic is the role of the notion of knowledge in practice. If one knows that A, then one is entitled to disregard in his thinking and acting all those scenarios in which A is not true. In an explicit semantics, these scenarios are called “possible worlds.” The notion of knowledge thus effects a dichotomy in the “space” of such possible worlds between those that are compatible with what one knows and those that are incompatible with it. The former are called one’s epistemic alternatives. This alternativeness relation (also called the “accessibility” relation) between possible worlds is the basis of the semantics of the logic of knowledge. In fact, the truth conditions for any epistemic proposition may be stated as follows: a person P knows that A if and only if it is the case that A is true in all of P’s epistemic alternatives. Asking what precisely the accessibility relation is amounts to asking what counts as being entitled to disregard the ruled-out scenarios, which itself is tantamount to asking for a definition of knowledge. Most of epistemic logic is nevertheless independent of any detailed definition of knowledge, as long as it effects a dichotomy of the kind indicated.

The logic of other epistemological notions is likewise based on other dichotomies between admitted and excluded possible worlds. For example, the scenarios excluded by one’s memory are those that are incompatible with what one remembers.

The basic notion of epistemic logic in the narrow sense is thus “knowing that.” In symbolic notation, “P knows that A” is usually expressed by K_PA. One of the aims of epistemic logic is to show how this construction can serve as the basis of other constructions. For example, “P knows whether A or B” can be expressed as (K_PA ∨ K_PB). “P knows who satisfies the condition A[x],” where A[x] does not contain any occurrences of K or any quantifiers, can be expressed as (∃x)K_PA[x]. Such a construction is called a simple wh-construction.

Epistemic logic is an example of intensional logic. Such logics are characterized by the failure of two of the basic laws of first-order logic, substitutivity of identity and existential generalization. The former authorizes an inference from an identity (a=b) and from a sentence A[a] containing occurrences of “a” to a sentence A[b], where some (or all) of those occurrences are replaced by “b.” The latter authorizes an inference from a sentence A[b] containing a constant b to the corresponding existential sentence (∃x)A[x]. The semantics of epistemic logic shows why these inference patterns fail and how they can be restored by an additional premise. Substitutivity of identity fails because, even though (a=b) is actually true, it may not be true in some of one’s epistemic alternatives, which is to say that the person in question (P) does not know that (a=b). Naturally, the inference from A[a] to A[b] may then fail, and, equally naturally, it is restored by an extra premise that says that P knows that a is b, or symbolically K_P(a=b). Thus, P may know that Anthony Eden was the British prime minister in 1956 but fail to know the same of Lord Avon, unless P happens to know that they are the same person.

Existential instantiation may fail even though something is true about an individual in all of P’s epistemic alternatives, the reason being that the individual (a) may be different in different alternatives. Then P does not know of any particular individual what he knows of a. The inference obviously goes through if P knows who or what a is—in other words, if it is true that (∃x)K_P(a=x). For example, P may know that Mary was murdered by Jack the Ripper and yet fail to know who she was murdered by—viz., if P (presumably like most people) does not know who Jack the Ripper is. These modifications of the laws of the substitutivity of identity and existential generalization are the characteristic features of epistemic logic.

It has turned out that not all knowledge constructions can be analyzed in this way in an epistemic logic whose only element that is not contained in first-order logic is the “knows that” operator. Such an analysis is impossible when the variable representing the entity that is supposed to be known depends on another variable. This is illustrated by knowing the result of a controlled experiment, which means knowing how the observed variable depends on the controlled variable. What is needed in order to make such constructions expressible is the notion of logical (informational) independence. For example, when the sentence (∃x)K_PA[x] is evaluated for its truth-value, it is not important that a value of x in (∃x) is chosen before one considers one of the epistemic P-alternatives. What is crucial is that the right value of x can be chosen independently of this alternative scenario. This kind of independence can be expressed by writing the existential quantifier as (∃x/K). This notation, known as the slash notation, enables one to express all the different knowledge constructions. For example, the outcome of a controlled experiment can be expressed in the form K(∀x)(∃y/K)A[x,y]. Simple wh-constructions such as (∃x)K_PA[x] can now be expressed by K_P(∃x/K_P)A[x] and the “whether” construction by K_P(A (∨/K_P) B).

One important distinction that can be made by means of slash notation is that between knowledge about propositions and knowledge about objects. In the former kind of knowledge, the slash is attached to a disjunction sign, as in (∨/K), whereas in the latter it is attached to an existential quantifier, as in (∃x/K). For example, “I know whether Tom murdered Dick” is symbolized as K_I(M(t,d) (∨/K_I) ~ M(t,d)), where M(x,y) is a shorthand for “x murdered y.” In contrast, “I know who murdered Dick” is symbolized by K_I(∃x/K_IM(x,d)).

It is often maintained that one of the principles of epistemic logic is that whatever is known must be true. This amounts to the validity of inferences from K_PA to A. If the knower is a deductively closed database or an axiomatic theory, this means assuming the consistency of the database or system. Such assumptions are known to be extremely strong. It is therefore an open question whether any realistic definition of knowledge can impose so strong a requirement on this concept. For this reason, it may in fact be advisable to think of epistemic logic as the logic of information rather than the logic of knowledge in this philosophically strong sense.

Two varieties of epistemic logic are often distinguished from each other. One of them, called “external,” is calculated to apply to other persons’ knowledge or belief. The other, called “internal,” deals with an agent’s own knowledge or belief. An epistemic logic of the latter kind is also called an autoepistemic logic.

An important difference between the two systems is that an agent may have introspective knowledge of his own knowledge and belief. Autoepistemic logic, therefore, contains a greater number of valid principles than external epistemic logic. Thus, a set Γ specifying what an agent knows will have to satisfy the following conditions: (1) Γ is closed with respect to logical consequence; (2) if A ∊ Γ, then KA ∊ Γ; (3) if A ∉ Γ, then ~KA ∊ Γ. Here K may also be thought of as a belief operator and Γ may be called a belief set. The three conditions (1)–(3) define what is known as a stable belief set. The conditions may be thought of as being satisfied because the agent knows what he knows (or believes) and also what he does not know (or believe).