Logic

Logic, the study of correct reasoning, especially as it involves the drawing of inferences.

This article discusses the basic elements and problems of contemporary logic and provides an overview of its different fields. For treatment of the historical development of logic, see logic, history of. For detailed discussion of specific fields, see the articles applied logic, formal logic, modal logic, and logic, philosophy of.

Scope and basic concepts

An inference is a rule-governed step from one or more propositions, called premises, to a new proposition, usually called the conclusion. A rule of inference is said to be truth-preserving if the conclusion derived from the application of the rule is true whenever the premises are true. Inferences based on truth-preserving rules are called deductive, and the study of such inferences is known as deductive logic. An inference rule is said to be valid, or deductively valid, if it is necessarily truth-preserving. That is, in any conceivable case in which the premises are true, the conclusion yielded by the inference rule will also be true. Inferences based on valid inference rules are also said to be valid.

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(Read Steven Pinker’s Britannica entry on rationality.)

Logic in a narrow sense is equivalent to deductive logic. By definition, such reasoning cannot produce any information (in the form of a conclusion) that is not already contained in the premises. In a wider sense, which is close to ordinary usage, logic also includes the study of inferences that may produce conclusions that contain genuinely new information. Such inferences are called ampliative or inductive, and their formal study is known as inductive logic. They are illustrated by the inferences drawn by clever detectives, such as the fictional Sherlock Holmes.

The contrast between deductive and ampliative inferences may be illustrated in the following examples. From the premise “somebody envies everybody,” one can validly infer that “everybody is envied by somebody.” There is no conceivable case in which the premise of this inference is true and the conclusion false. However, when a forensic scientist infers from certain properties of a set of human bones the approximate age, height, and sundry other characteristics of the deceased person, the reasoning used is ampliative, because it is at least conceivable that the conclusions yielded by it are mistaken.

In a still narrower sense, logic is restricted to the study of inferences that depend only on certain logical concepts, those expressed by what are called the “logical constants” (logic in this sense is sometimes called elementary logic). The most important logical constants are quantifiers, propositional connectives, and identity. Quantifiers are the formal counterparts of English phrases such as “there is …” or “there exists …,” as well as “for every …” and “for all …” They are used in formal expressions such as (∃x) (read as “there is an individual, call it x, such that it is true of x that …”) and (∀y) (read as “for every individual, call it y, it is true of y that …”). The basic propositional connectives are approximated in English by “not” (~), “and” (&), “or” (∨ ), and “if … then …” (⊃). Identity, represented by ≡, is usually rendered in English as “… is …” or “… is identical to …” The two example propositions above can then be expressed as (1) and (2), respectively:

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(1) (∃x)(∀y) (x envies y)

(2) (∀y)(∃x) (x envies y)

The way in which the different logical constants in a proposition are related to each other is known as the proposition’s logical form. Logical form can also be thought of as the result of replacing all of the nonlogical concepts in a proposition by logical constants or by general logical symbols known as variables. For example, by replacing the relational expression “a envies b” by “E(a,b)” in (1) and (2) above, one obtains (3) and (4), respectively:

(3) (∃x)(∀y) E(x,y)

(4)(∀y)(∃x) E(x,y)

The formulas in (3) and (4) above are explicit representations of the logical forms of the corresponding English propositions. The study of the relations between such uninterpreted formulas is called formal logic.

It should be noted that logical constants have the same meaning in logical formulas, such as (3) and (4), as they do in propositions that also contain nonlogical concepts, such as (1) and (2). A logical formula whose variables have been replaced by nonlogical concepts (meanings or referents) is called an “interpreted” proposition, or simply an “interpretation.” One way of expressing the validity of the inference from (3) to (4) is to say that the corresponding inference from a proposition like (1) to a proposition like (2) will be valid for all possible interpretations of (3) and (4).

Valid logical inferences are made possible by the fact that the logical constants, in combination with nonlogical concepts, enable a proposition to represent reality. Indeed, this representational function may be considered their most fundamental feature. A proposition G, for example, can be validly inferred from another proposition F when all of the scenarios represented by F—the scenarios in which F is true—are also scenarios represented by G—the scenarios in which G is true. In this sense, (2) can be validly inferred from (1) because all of the scenarios in which it is true that someone envies everybody are also scenarios in which it is true that everybody is envied by at least one person.

A proposition is said to be logically true if it is true in all possible scenarios, or “possible worlds.” A proposition is contradictory if it is false in all possible worlds. Thus, another way to express the validity of the inference from F to G is to say that the conditional proposition “If F, then G” (F ⊃ G) is logically true.

Not all philosophers accept these explanations of logical validity, however. For some of them, logical truths are simply the most general truths about the actual world. For others, they are truths about a certain imperceptible part of the actual world, one that contains abstract entities like logical forms.

In addition to deductive logic, there are other branches of logic that study inferences based on notions such as knowing that (epistemic logic), believing that (doxastic logic), time (tense logic), and moral obligation (deontic logic), among others. These fields are sometimes known collectively as philosophical logic or applied logic. Some mathematicians and philosophers consider set theory, which studies membership relations between sets, to be another branch of logic.