In philosophy, theory (from ancient Greek theoria, θεωρία, meaning "a looking at, viewing, beholding") refers to contemplation or speculation, as opposed to action. Theory is especially often contrasted to "practice" (Greek praxis, πρᾶξις) a concept that in its original Aristotelian context referred to actions done for their own sake, but can also refer to "technical" actions instrumental to some other aim, such as the making of tools or houses. "Theoria" is also a word still used in theological contexts.

A classical example uses the discipline of medicine to explain the distinction: Medical theory and theorizing involves trying to understand the causes and nature of health and sickness, while the practical side of medicine is trying to make people healthy. These two things are related but can be independent, because it is possible to research health and sickness without curing specific patients, and it is possible to cure a patient without knowing how the cure worked.

The verb θεωρία apparently developed special uses early in the Greek language. In the book, From Religion to Philosophy, Francis Cornford suggests that the Orphics used the word "theory" to mean 'passionate sympathetic contemplation' . Pythagoras changed the word to mean a passionate sympathetic contemplation of mathematical and scientific knowledge. This was because Pythagoras considered such intellectual pursuits the way to reach the highest plane of existence. Pythagoras stressed on killing the emotions and the lusts of the body and the release of the intellect to soar into the exalted domain of theory. Thus it was Pythagoras who gave the word "theory" the specific meaning which leads to the classical and modern concept of a distinction between theory as uninvolved, neutral thinking, and practice.

While theories in the arts and philosophy may address ideas and not easily observable empirical phenomena, in modern science the term "theory", or "scientific theory" is generally understood to refer to a proposed explanation of empirical phenomena, made in a way consistent with the scientific method. Such theories are preferably described in such a way that any scientist in the field is in a position to understand, verify, and challenge (or "falsify") it. In this modern scientific context the distinction between theory and practice corresponds roughly to the distinction between theoretical science and technology or applied science.

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

• Fact is not enough, opinion is too much.
  • Todor Simeonov

• 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

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Thu Aug 19 21:09:23 2010