Is it true that in science 'theoretical' means 'non-empirical'? If so, are theoretical entities radically imperceptible? That is, although we can perceive the effects of theoretical entities, we can never perceive the entities themselves. For example, theoretical temperature is average kinetic energy of molecules, which we cannot perceive, but we can perceive its effects as thermometer readings and sensations of hot and cold; or mass is imperceptible but we can perceive its effects as forces of weight and inertia.

Sometimes, philosophers use the term "theoretical" to apply to certain statements in a scientific theory. Sometimes, they use the term to apply to certain entities whose existence is postulated by a theory, viz., those entities that are not directly observable. In the latter sense, they are contrasted not so much to "non-empirical" entities, but to observable ones. There is a lot of dispute about what "not directly observable" means. Are entities that we can see only with a telescope "directly observable"? Only with a microscope? Only with my glasses on? This kind of continuum has led some philosophers to declare that all entities are in principle observable. And others to hold that no entities are directly observable except "sense data", categorically unmediated sensory experiences. If you do think there are theoretical entities, or that most mature sciences contain statements with terms purporting to refer to them, then a major issue in the philosophy of science is what to make of (how to understand) such statements. Are they fully meaningful? How are we to understand their meaning? Are they true or false? Can we have evidence for their truth?

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