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Theory has a number of distinct meanings in different fields of knowledge, depending on the context and their methodologies.



The word ‘theory’ derives from the Greek ‘theorein’, which means ‘to look at’. According to some sources, it was used frequently in terms of ‘looking at’ a theatre stage, which may explain why sometimes the word ‘theory’ is used as something provisional or not completely resembling real. The term ‘theoria’ (a noun) was already used by the scholars of ancient Greeks.


In scientific usage, theory is not the opposite of fact. Theories are typically ways of explaining why things happen, usually after the fact that they happen is no longer in scientific dispute. In referring to the "theory of global warming", for example, there is no implication that global warming is not occurring; world temperatures have been measured and are increasing. The "theory of global warming" refers instead to scientific work that explains how and why this has been happening.

In various sciences, a theory is a logically self-consistent model or framework for describing the behavior of a certain natural or social phenomenon, thus either originating from observable facts or supported by them (see scientific method). In this sense, a theory is a systematic and formalized expression of all previous observations made that is predictive, logical, testable, and has never been falsified.

In physics, the term theory is generally used for a mathematical framework derived from a small set of basic principles, capable of producing experimental predictions for a given category of physical systems. A good example is electromagnetic theory, which encompasses the results that can be derived from Maxwell's equations. This theory is usually taken to be synonymous with classical electromagnetism.

The term theoretical is used in science to describe a result that is predicted by theory but has not yet been observed. For example, until recently, black holes were considered theoretical. It is not uncommon in the history of physics for theory to produce predictions that are later confirmed by experiment; failed predictions, however, also occur, and sometimes work to falsify a theory. Conversely, at any time in the study of physics there can also be confirmed experimental results that are not yet explained by theory.

For a given body of theory to be considered part of established scientific knowledge, it is usually necessary for it to characterize a critical experiment, namely an experimental result not predicted by any existing established theory.

Unfortunately, the usage of the term theory is muddled by cases such as string theory and various theories of everything, all of which are better characterized at present as a bundle of competing hypotheses or a protoscience. A hypothesis, however, is still vastly more reliable than a conjecture, which is at best an untested guess consistent with selected data and often simply a belief based on non-repeatable experiments, anecdotes, popular opinion, "wisdom of the ancients," commercial motivation, or mysticism.

Claims such as intelligent design and homeopathy are not considered by mainstream scientists to be scientific theories, but pseudoscience.


Humans construct theories in order to explain, predict and master phenomena (e.g. inanimate things, events, or the behaviour of animals). In many instances, this is seen to be the construction of models of reality. A theory makes generalizations about observations and consists of an interrelated, coherent set of ideas and models.

According to Stephen Hawking in A Brief History of Time, "a theory is a good theory if it satisfies two requirements: It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations." He goes on to state, "any physical theory is always provisional, in the sense that it is only a hypothesis; you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory."

Types of theories

There are two uses of the word theory; a supposition which is not backed by observation is known as a conjecture, and if backed by observation it is a hypothesis. Most theory evolves from hypotheses, but the reverse is not true: many hypotheses turn out to be false and so do not evolve into theory.

A theory is different from a theorem. The former is a model of physical events and cannot be proved from basic axioms. The latter is a statement of mathematical fact which logically follows from a set of axioms. A theory is also different from a physical law in that the former is a model of reality whereas the latter is a statement of what has been observed.

Theories can become accepted if they are able to make correct predictions and avoid incorrect ones. Theories which are simpler, and more mathematically elegant, tend to be accepted over theories which are complex. Theories are more likely to be accepted if they connect a wide range of phenomena. The process of accepting theories, or of extending existing theory, is part of the scientific method.

Further explanation of a scientific theory

In common usage a theory is often viewed as little more than a guess or a hypothesis. But in science and generally in academic usage, a theory is much more than that. A theory is an established paradigm that explains all or much of the data we have and offers valid predictions that can be tested. In science, a theory is never considered fact or infallible, because we can never assume we know all there is to know. Instead, theories remain standing until they are disproven, at which point they are thrown out altogether or modified to fit the additional data.

Theories start out with empirical observations such as "sometimes water turns into ice." At some point, there is a need or curiosity to find out why this is, which leads to a theoretical/scientific phase. In scientific theories, this then leads to research, in combination with auxiliary and other hypotheses (see scientific method), which may then eventually lead to a theory. Some scientific theories (such as the theory of gravity) are so widely accepted that they are often seen as laws. This, however, rests on a mistaken assumption of what theories and laws are. Theories and laws are not rungs in a ladder of truth, but different sets of data. A law is a general statement based on observations.

A canonical example of a disproven theory is the geocentric model of the universe proposed by Ptolemy. Evidence, in the form of Galileo's observation of the phases of Venus in 1610, was produced which was completely incompatible with the predictions set forth by the theory. This falsification, though, did not necessarily mean that only one alternative theory was necessarily the "correct" replacement — both the Copernican system and the Tychonian system predicted the phases of Venus.


There is sometimes confusion between the scientific use of the word theory and its more informal use as a synonym for "speculation" or "conjecture." In science, a body of descriptions of knowledge is usually only called a theory once it has a firm empirical basis, i.e., it

  1. is consistent with pre-existing theory to the extent that the pre-existing theory was experimentally verified, though it will often show pre-existing theory to be wrong in an exact sense,
  2. is supported by many strands of evidence rather than a single foundation, ensuring that it probably is a good approximation if not totally correct,
  3. has survived many critical real world tests that could have proven it false,
  4. makes predictions that might someday be used to disprove the theory,
  5. is tentative, correctable and dynamic, in allowing for changes to be made as new data is discovered, rather than asserting certainty, and
  6. is the most parsimonious explanation, sparing in proposed entities or explanations, commonly referred to as passing Occam's Razor.

This is true of such established theories as special and general relativity, quantum mechanics, plate tectonics, evolution, etc. Theories considered scientific meet at least most, but ideally all, of the above criteria. The fewer which are matched, the less scientific it is; those that meet only several or none at all, cannot be said to be scientific in any meaningful sense of the word.


In mathematics, the word theory is used informally to refer to certain distinct bodies of knowledge about mathematics. This knowledge consists of axioms, definitions, theorems and computational techniques, all related in some way by tradition or practice. Examples include group theory, set theory, Lebesgue integration theory and field theory.

The term "theory" also has a formal usage in mathematics, particularly in mathematical logic and model theory. A theory in this sense is a set of statements closed under certain rules of inference. A typical theory will present certain axioms and rules, corresponding to a useful or interesting abstraction, and then derive non-obvious theorems from those axioms. The resulting theorems often provide solutions to real-world problems which correspond to the original abstraction. Obvious examples include arithmetic (abstracting the concept of number), geometry (the concept of space), and probability (the concept of randomness).

However, Gödel's incompleteness theorem shows that no consistent theory capable of defining the concept of natural numbers can derive all true statements about those numbers. This sets a fundamental limit to the applicability of any mathematical system.

Other fields

Theories exist not only in the so-called hard sciences; but in all fields of academic study, from philosophy to music to literature. In the humanities, theory is often used as an abbreviation for critical theory or literary theory, referring to continental philosophy's aesthetics or its attempts to understand the structure of society and to conceptualize alternatives. In philosophy, theoreticism refers to the overuse of theory.

List of famous theories

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