theoretical term a term occurring in a scientific theory that purports to make reference to an unobservable entity (e.g., ‘electron’), property (e.g., ‘the monatomicity of a molecule’), or relation (‘greater electrical resistance’). The qualification ‘purports to’ is required because instrumentalists deny that any such unobservables exist; nevertheless, they acknowledge that a scientific theory, such as the atomic theory of matter, may be a useful tool for organizing our knowledge of observables and predicting future experiences. Scientific realists, in contrast, maintain that at least some of the theoretical terms (e.g., ‘quark’ or ‘neutrino’) actually denote entities that are not directly observable – they hold, i.e., that such things exist. For either group, theoretical terms are contrasted with such observational terms as ‘rope’, ‘smooth’, and ‘louder than’, which refer to observable entities, properties, or relations. Much philosophical controversy has centered on how to draw the distinction between the observable and the unobservable. Did Galileo observe the moons of Jupiter with his telescope? Do we observe bacteria under a microscope? Do physicists observe electrons in bubble chambers? Do astronomers observe the supernova explosions with neutrino counters? Do we observe ordinary material objects, or are sense-data the only observables? Are there any observational terms at all, or are all terms theory-laden? Another important meaning of ‘theoretical term’ occurs if one regards a scientific theory as a semiformal axiomatic system. It is then natural to think of its vocabulary as divided into three parts, (i) terms of logic and mathematics, (ii) terms drawn from ordinary language or from other theories, and (iii) theoretical terms that constitute the special vocabulary of that particular theory. Thermodynamics, e.g., employs (i) terms for numbers and mathematical operations, (ii) such terms as ‘pressure’ and ‘volume’ that are common to many branches of physics, and (iii) such special thermodynamical terms as ‘temperature’, ‘heat’, and ‘entropy’. In this second sense, a theoretical term need not even purport to refer to unobservables. For example, although special equipment is necessary for its precise quantitative measurement, temperature is an observable property.
Even if theories are not regarded as axiomatic systems, their technical terms can be considered theoretical. Such terms need not purport to refer to unobservables, nor be the exclusive property of one particular theory. In some cases, e.g., ‘work’ in physics, an ordinary word is used in the theory with a meaning that departs significantly from its ordinary use.
Serious questions have been raised about the meaning of theoretical terms. Some philosophers have insisted that, to be meaningful, they must be given operational definitions. Others have appealed to coordinative definitions to secure at least partial interpretation of axiomatic theories. The verifiability criterion has been invoked to secure the meaningfulness of scientific theories containing such terms.
A theoretical concept (or construct) is a concept expressed by a theoretical term in any of the foregoing senses. The term ‘theoretical entity’ has often been used to refer to unobservables, but this usage is confusing, in part because, without introducing any special vocabulary, we can talk about objects too small to be perceived directly – e.g., spheres of gamboge (a yellow resin) less than 10–6 meters in diameter, which figured in a historically important experiment by Jean Perrin.
See also OPERATIONALISM , PHILOSOPHY OF SCIENC. W.C.S.