In the philosophy of science, instrumentalism is the view that concepts and theories are merely useful instruments whose worth is measured not by whether the concepts and theories are true or false (or correctly depict reality), but how effective they are in explaining and predicting phenomena.Instrumentalism relates closely to pragmatism. This methodological viewpoint often contrasts with scientific realism, which defines theories as specially being more or less true. However, instrumentalism is more of a pragmatic approach to science, information and theories than an ontological statement. Oftentimes instrumentalists (just like pragmatists) have been accused of being relativists, even though many instrumentalists are also believers in sturdy objective realism (such as Karl Popper).
Instrumentalist morality thus resembles utilitarianism in defining moral rules only as tools for moral good. Thus the moral code rising from a given population is simply a collection of rules that are useful to the population. David Hume was perhaps the first person to suggest that there might not be any intrinsic or metaphysical value of rules, but that they are simply secular and natural rules that are human-made.
Political instrumentalism is a view first suggested by John Dewey and later by the Chicago school of economists, which sees politics as simply means to an end. Milton Friedman paraphrased the viewpoint by explaining that he had no ideological love for free markets, but he might as simply be a socialist if socialism fulfilled the ends most people seem to want. The fallibilistic epistemology of Karl Popper adds to this a belief that we should empirically measure all politics and verify whether or not they fulfill their goals, and try to falsify our politics, critique them and come up with better ways to reach the ends.
In the philosophy of mind, instrumentalism is the view (championed by Daniel Dennett), that propositional attitudes such as belief are not concepts on which we can base scientific investigations of the mind and brain, but that acting as if other beings do have beliefs is often a successful strategy. For example, acting as if the chess playing computer has the belief that taking the queen will give it a significant advantage is a successful strategy, despite the fact that few people would argue simple electronics devices have beliefs as we normally think of them.