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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 is primarily a mathematical model to describe the world on the quantum scale — that is, the scale ranging from sub-atomic particles up to molecules. This is something that it does very well, with only one problem; these equations defy common sense. This is the reason for the various interpretations of quantum theory, such as the "many worlds" theory or the Copenhagen Interpretation, which explain why there should be weird things like particle-wave duality. From the point of view of instrumentalism such interpretations are unnecessary as all quantum theory needs to do is correctly predict the behaviour of atoms and molecules.
Principle of least action
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.