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Philosophy of science
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Instrumentalism is the scientific philosophy that states that theories should be evaluated on how well they predict and model phenomena, and not how well they describe the Universe. This has implications for the various interpretations of quantum mechanics. It was advanced by John Dewey, a noted pragmatist.

The philosophy is often summed up by the David Mermin quote as "shut up and calculate!", indicating that battling over the interpretations of theory should be irrelevant next to their predictive power. Also summing up the idea was Niels Bohr, who stated: "Its wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature" By this, Bohr means science should just be a series of predictive statements that work to describe how nature reacts around us.

Quantum mechanics[edit]

See the main article on this topic: Quantum mechanics

Quantum mechanics is primarily a mathematical model to describe the outcome of experiments involving microscopic systems — that is, the scale ranging from sub-atomic particles up to molecules. This is something that it does very well, with only one problem; the equations in their normal formulation have the concept of measurement as an unexplained primitive and directly describe experimental outcomes not the microscopic systems themselves directly.

The various interpretations of quantum theory attempt to deal with this state of affairs. One class known, as Realist interpretations, attempt to describe what microscopic systems are in and of themselves and to explain what occurs during measurement while treating it no differently than any other interaction. Examples are the "many worlds" theory or Bohmian Mechanics. The other class, commonly called Instrumentalist (also Anti-Realist or Non-Representational), accept Quantum Mechanics as typically formulated. They regard the lack of a description of what microscopic systems are actually like as a fundamental feature of subatomic physics. One can only describe their effect on the macroscopic world (or agents), not an objective account of them in themselves. Examples of such views are the Copenhagen Interpretation and QBism.

Principle of least action[edit]

The concept of instrumentalism doesn't just apply to quantum theory, where we know strange things happen, but also to classical mechanics. The principle of least action is a part of mechanics which allows equations of motion to be derived. Put briefly and simply, the principle states that things will take the path of least resistance when moving. However, compared with other parts of classical mechanics the principle of least action seems to imply that objects must think and predict their course of motion, an apparent "teleology". From an instrumentalist interpretation, this doesn't particularly matter as the theory predicts motion fine and we shouldn't worry that it implies that objects can think ahead.


Notably Karl Popper was dead against instrumentalism, seeing it as far too mechanical and focused on mere computation:

…My reply to instrumentalism consists in showing that there are profound differences between "pure" theories and technological computation rules, and that instrumentalism can give a perfect description of these rules but is quite unable to account for the difference between them and the theories

Critics of instrumentalism point out that it must deny the truth value of theories, as the only thing science can provide, in instrumentalist philosophy, are data and predictions.