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It has been said, slightly imprecisely, that the operational definition of a cake is the recipe for baking the cake,which we can regard as a state preparation process. A similar saying, if it walks like a duck and quacks like a duck, it's a duck, can be regarded as a sort of measurement process.
Despite the controversial philosophical origins of the concept, particularly its close association with logical positivism, operational definitions have undisputed practical applications. Operational definitions are particularly important in quantum mechanics, statistical physics and relativity.
The idea originally arises in the operationalist philosophy of P. W. Bridgman and others. By 1914, Bridgman was dismayed by the abstraction and lack of clarity with which, he argued, many scientific concepts were expressed. Inspired by logical positivism and the phenomenalism of Ernst Mach, in 1914 he declared that the meaning of a physical concept, such as mass, lay in the operations, physical and mental, performed in its measurement.
Thus, mass, as measured with a spring balance, and mass, as measured using a load cell are quite distinct concepts. That they are highly correlated in general is interesting, and worth noting, but ought not lead us to abstract discussions about the mass or the true mass of a body.
He developed his philosophy in his book The Logic of Modern Physics ( 1927).
Physical quantities, such as temperature and electric current are commonly defined in textbooks in terms of their abstract definitions (vide infra). This leads to some practical difficulties for the standardisation demanded for tradeTrade centers on the exchange of goods and/or services. Exchanges may take place between two parties (bilateral trade) or amongst more than two parties (multilateral trade). In its original form trade necessarily used barter and the exchange of goods and and for testing the reproducibilityReproducibility is one of the main principles of the scientific method. An experimental description ( thought experiment) produced by a particular researcher or group of researchers is generally evaluated by other independent researchers by attempting to of scientific results. Standardisation bodies, therefore, specify physical quantities in terms of operational definitions in order to facilitate agreement and reproducibility.
Operational definitions are at their most controversial in the field of psychologyPsychology is the study of mind, thought, and behaviour. It is largely concerned with humans, although the behaviour and thought of animals is also studied; either as a subject in its own right (see animal cognition), or more controversially, as a way of where intuitive concepts, such as intelligenceIntelligence is a general mental capability that involves the ability to reason, plan, solve problems, think abstractly, comprehend ideas and language, and learn. While the definition and importance of intelligence is an issue of some controversy, especia need to be operationally definied before they become amenable to scientific investigation, for example, through processes such as IQ tests. Such definitions are used as a follow up to a conceptual definition, in which the specific concept is defined as a measurable occurrence. John Stuart Mill pointed out the dangers of believing that anything that could be given a name must refer to a thing and Stephen Jay Gould and others have criticised psychologists for doing just that. A committed operationalist would respond that speculation about the thing in itself should be resisted and comment only made on the tables of operationally defined measurements.