Theory is a contemplative and rational type of abstract or generalizing thinking, or the results of such thinking. Depending on the context, the results might for example include generalized explanations of how nature works, or even how divine or metaphysical matters are thought to work. The word has its roots in ancient Greek, but in modern use it has taken on several different related meanings.

One modern group of meanings emphasizes the speculative and generalizing nature of theory. For example in the arts and philosophy, the term "theoretical" may be used to describe ideas and empirical phenomena which are not easily measurable. And by extension of the philosophical meaning, "theoria" is also a word still used in theological contexts. As already in Aristotle's definitions, theory is very often contrasted to "practice" (from Greek praxis, πρᾶξις) a Greek term for "doing", which is opposed to theory because pure theory involves no doing apart from itself. A classical example of the distinction between theoretical and practical uses the discipline of medicine: medical theory 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.

In modern science, the term "theory" refers to scientific theories, a well-confirmed type of explanation of nature, made in a way consistent with scientific method, and fulfilling the criteria required by modern science. Such theories are described in such a way that any scientist in the field is in a position to understand and either provide empirical support ("verify") or empirically contradict ("falsify") it. Scientific theories are the most reliable, rigorous, and comprehensive form of scientific knowledge, in contrast to more common uses of the word "theory" that imply that something is unproven or speculative (which is better defined by the word 'hypothesis'). Scientific theories are also distinguished from hypotheses, which are individual empirically testable conjectures, and scientific laws, which are descriptive accounts of how nature will behave under certain conditions.

Read more about Theory:  Ancient Uses, Theories Formally and Scientifically, Philosophical Theories, Scientific Theories, List of Notable Theories

Famous quotes containing the word theory:

    Thus the theory of description matters most.
    It is the theory of the word for those
    For whom the word is the making of the world,
    The buzzing world and lisping firmament.
    Wallace Stevens (1879–1955)

    In the theory of gender I began from zero. There is no masculine power or privilege I did not covet. But slowly, step by step, decade by decade, I was forced to acknowledge that even a woman of abnormal will cannot escape her hormonal identity.
    Camille Paglia (b. 1947)

    By the “mud-sill” theory it is assumed that labor and education are incompatible; and any practical combination of them impossible. According to that theory, a blind horse upon a tread-mill, is a perfect illustration of what a laborer should be—all the better for being blind, that he could not tread out of place, or kick understandingly.... Free labor insists on universal education.
    Abraham Lincoln (1809–1865)