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.

The differences between fact and theory and between theory and practice are a common concern of scientists, engineers, and philosophers.

• Facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts do not go away while scientists debate rival theories for explaining them. Einstein's theory of gravitation replaced Newton's, but apples did not suspend themselves in mid-air pending the outcome.
  • Stephen Jay Gould, 1981

• Facts are meaningless. You could use facts to prove anything that's even remotely true!
  • Homer Simpson

• In theory, there is no difference between theory and practice. But, in practice, there is.
  • Jan L. A. van de Snepscheut/Yogi Berra. Also cited in Applying Use Case Driven Object Modeling With UML, Doug Rosenberg and Kendall Scott p.1 [1]

• Facts were never pleasing to him. He acquired them with reluctance and got rid of them with relief. He was never on terms with them until he had stood them on their heads.
  • J.M. Barrie, The Greenwood Hat (1937)

• But facts are chiels that winna ding, and downa be disputed.
  • Robert Burns, A Dream (1786)

• There is no great harm in the theorist who makes up a new theory to fit a new event. But the theorist who starts with a false theory and then sees everything as making it come true is the most dangerous enemy of human reason.
  • Gilbert Keith Chesterton The Flying Inn

From Wikiquote under the GNU Free Documentation License.
Tue Sep 1 03:16:11 2009