Pragmatism, Relativism and Quantum Theory

We can understand quantum theory better if we stop asking how it represents the world and ask instead how we are able to use it so successfully in predicting, explaining and controlling the world.

In a pragmatist view, quantum theory helps us to do these things by acting as a source of reliable advice about what to believe and so how to act in situations where we are not in a position to know what happens. The function of Born probabilities is to guide our credences (coherent degrees of belief), and the primary role of a quantum state is to yield Born probabilities. Both are relational because their advice depends on what information is accessible from the situation of any agent seeking it. But the objective facts about quantum states and probabilities are not relative to anything. Recent no-go results show that in extended Wigner’s friend scenarios (EWFS) the outcome of a measurement would be a merely relative fact. But this does not undercut the objectivity of the measurement outcomes we take to warrant acceptance of quantum theory.