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A theoretical (or conceptual) definition gives the meaning of a word in terms of the theories of a specific discipline. This type of definition assumes both knowledge and acceptance of the theories that it depends on. To theoretically define is to create a hypothetical construct. This method of operationalization is not to be confused with operationally defining. An example of a theoretical definition is that of "Heat" in physics, which actually puts forth an entire theory of heat (involving accelerating molecules, etc.).
Theoretical definitions are common in both philosophy and science, and can be difficult to understand because of their strict, and often conceptual uses. The goal is to eliminate vagueness (e.g. how many metres exactly is a "tall" person?) and ambiguity (e.g. "I purchased a bat" could have many meanings). Theoretical definitions specify exactly when the word should and should not be applied. In this regard they are unlike persuasive definitions, which can be both vague and ambiguous. Theoretical definitions do, however, have one thing in common with persuasive definitions: they are normative, and not merely descriptive. To create a theoretical definition is to propose a way of thinking about an issue. Indeed, theoretical definitions contain built-in theories; they cannot be simply reduced to describing a set of observations. The definition will contain implicit inductions and deductive consequences that are part of the theory it pushes. The word "Heat" in physics is not simply describing molecules, it is proposing various laws of nature and predicting certain results.
Like Stipulative definitions, it is not a valid criticism to say that a theoretical definition is "wrong about how most people use the word" nor that "the definition itself is false". Instead one might say that a theoretical definition is unhelpful. This is unlike lexical definitions, which themselves claim to be common, popular uses of a word. In contrast, a theoretical definition is only a bad one if the theories that it supports are invalidated or falsified (which occurs through conflict with other theories that have been accepted). It is at that point - once they describe falsified theories - that these kinds of definitions become unhelpful or 'unpromising' (e.g. the theoretical definition of "demonic possession" is not itself false, the ideas are, and so the term is useless to modern medicine). It is in this way that professional fields build frameworks of agreed-upon theoretical definitions. A case in point, consider "Heat". If a physicist's theories of molecules turn out to be wrong in some sense, this would make "Heat" an unhelpful theoretical definition, not a false one (i.e. this kind of definition only invites us to use certain theories, and the theories make the truth claims). Moreover, like the theories that build them, theoretical definitions change as scientific understanding grows.
Psychology has many examples of ideas that required conceptual definitions, including intelligence, knowledge, tolerance, and preference. Following the establishment of a theoretical definition, the researcher must use an operational definition to indicate how the abstract concept will be measured.
|Theoretical or Conceptual definition||Operational definition|
|Weight: a measurement of gravitational force acting on an object||a result of measurement of an object on a Newton spring scale|
Theoretical definitions are common in scientific contexts, where theories tend to be more precisely defined, and results are more widely accepted as correct. The definitions of substances as various configurations of atoms are theoretical definitions, as are definitions of colors as specific wavelengths of reflected light. In such cases one definition of a term is unlikely to contradict another definition based on a different theory. However, in areas such as philosophy and the social sciences, theoretical definitions of the same term often contradict each other depending on whose theory is being used as the basis.
Another example of a theoretical definition: the length of a metre is "the distance traveled by light in a vacuum during a time interval of 1/299,792,458 of a second". This is based on the first postulate of special relativity theory that the speed of light in vacuum is the same to all inertial observers (i.e. it is a constant, and therefore a good measure of length). Thus we have defined 'metre' according to other ideas contained in modern scientific theory. Rejection of the theory underlying a theoretical definition leaves the definition invalid for use in argument with those who reject it — neither side will advance its position by using terms the others do not accept.
For example, John Searle's Chinese room thought experiment illustrates how differing theoretical definitions of "thinking" have caused conflict amongst artificial intelligence philosophers. Some philosophers might call "thought" merely "having the ability to convince another person that you can think". An accompanying operational definition for this theoretical definition could be a simple conversation test (e.g. Turing test). In contrast, Searle believes that better theoretical and operational definitions are required.
A recent effect of differing theoretical definitions occurred when millions of Americans went from normal to overweight in a day's time. The change in the theoretical definition of "overweight" was based on new theories put forth by the National Institutes of Health suggesting greater risks than originally believed. The government then changed the operational definition of "overweight" to "having a BMI over 25" (rather than 27 for women and 28 for men). Changing our understanding of when someone is "overweight" put several million more in that category, even though they had not actually gained or lost any weight.