A theory, in the general sense of the word, is an analytic structure designed to explain a set of observations. A 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.

The term is often used colloquially to refer to any suggested explanation, even fanciful or speculative ones, for an event or class of events. In scholarly use, however, the term is reserved for ideas which meet baseline requirements about the kinds of 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 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.
Thu Jun 11 06:04:00 2009