Noun

Singular theory

Plural countable and uncountable; plural theories

theory (countable and uncountable; plural theories)

1. (countable) An unproven conjecture.

   I have a theory about who broke into the school last night, but I have no proof to back it up.

2. (uncountable) An expectation of what should happen, barring unforeseen circumstances.

   So we’ll be there in three hours? — That’s the theory.

3. (countable) (sciences) A coherent statement or set of statements that attempts to explain observed phenomena.

   There is now a well-developed theory of electrical charge.

4. (countable) (sciences) A logical structure that enables one to deduce the possible results of every experiment that falls within its purview.

   The theory of relativity was proposed by Einstein.

5. (uncountable) (mathematics) A field of study attempting to exhaustively describe a particular class of constructs.

   Knot theory classifies the mappings of a circle into 3-space.

6. (countable) (logic) A set of axioms together with all statements derivable from them.

   A theory is consistent if it has a model.

Synonyms

• See also Wikisaurus:supposition

Derived terms

• acoustic theory
• algorithmic information theory
• antenna theory
• atomic theory
• catastrophe theory
• category theory
• cell theory
• chaos theory
• circuit theory
• complexity theory
• computation theory
• control theory
• critical theory
• decision theory

• domino theory
• extreme value theory
• game theory
• giant impact theory
• graph theory
• group theory
• in theory
• information theory
• kinetic theory of gases
• knot theory
• literary theory
• music theory
• number theory
• phlogiston theory

• probability theory
• proof theory
• quantum field theory
• rational choice theory
• set theory
• signal theory
• social theory
• systems theory
• theory of gravity
• theory of relativity
• theory of truth
• type theory
• value theory
• virtue theory

Related terms

• theorem
• theoretical
• theorise, theorize

The term theory has two broad sets of meanings, one used in the empirical sciences (both natural and social) and the other used in philosophy, mathematics, logic, and across other fields in the humanities. There is considerable difference and even dispute across academic disciplines as to the proper usages of the term. What follows is an attempt to describe how the term is used, not to try to say how it ought to be used.

Although the scientific meaning is by far the more commonly used in academic discourse, it is hardly the only one used, and it would be a mistake to assume from the outset that a given use of the term "theory" in academic literature or discourse is a reference to a scientific or empirically-based theory.

Even so, since the use of the term theory in scientific or empirical inquiry is the more common one, it will be discussed first. (Other usages follow in the section labeled "Theories formally and generally.")

A theory, in the scientific sense of the word, is an analytic structure designed to explain a set of empirical observations. A scientific theory does two things:

1. it identifies this set of distinct observations as a class of phenomena, and
2. makes assertions about the underlying reality that brings about or affects this class.

In the scientific or empirical tradition, the term "theory" is reserved for ideas which meet baseline requirements about the kinds of empirical observations made, the methods of classification used, and the consistency of the theory in its application among members of the class to which it pertains. These requirements vary across different scientific fields of knowledge, but in general theories are expected to be functional and parsimonious: i.e. a theory should be the simplest possible tool that can be used to effectively address the given class of phenomena.

Theories are distinct from theorems: theorems are derived deductively from theories according to a formal system of rules, generally as a first step in testing or applying the theory in a concrete situation. Theories are abstract and conceptual, and to this end they are never considered right or wrong. Instead, they are supported or challenged by observations in the world. They are 'rigorously tentative', meaning that they are proposed as true but expected to satisfy careful examination to account for the possibility of faulty inference or incorrect observation. Sometimes theories are falsified, meaning that an explicit set of observations contradicts some fundamental assumption of the theory, but more often theories are revised to conform to new observations, by restricting the class of phenomena the theory applies to or changing the assertions made. Sometimes a theory is set aside by scholars because there is no way to examine its assertions analytically; these may continue on in the popular imagination until some means of examination is found which either refutes or lends credence to the theory.

The word 'theory' is generally considered to derive from Greek θεωρία theoria (Jerome), Greek "contemplation, speculation", from θεωρός "spectator", θέα thea "a view" + ὁρᾶν horan "to see", literally "looking at a show". A second possible etymology traces the word back to το θείον to theion "divine things" instead of thea, reflecting the concept of contemplating the divine organisation (Cosmos) of the nature. The word has been in use in English since at least the late 16th century.