Abstract

On a pragmatist view of quantum theory, a quantum state has the role of advising physically situated agents rather than representing the condition of physical systems. The advice concerns the cognitive significance of a magnitude claim S: σ has, locating the value of magnitude Q on system σ in set Δ of real numbers. The quantum state offers advice both on the content of a magnitude claim S and on its credibility, provided it has enough content. The advice is authoritative—anyone who both accepts quantum theory and agrees on the correct quantum state is bound to heed it. On this view, the content of a magnitude claim is a function of its place in a web of material inferences connecting it to other claims, and hence to perception and action. A quantum state offers advice on the content of a magnitude claim by controlling its place in this inferential web. It thereby adds a contextual element to the content even of claims about the properties of familiar objects like gross experimental apparatus and the moon. But by modeling the behavior of quantum states, quantum theory itself reassures us that only for claims about currently unfamiliar objects does the consequent modification of content amount to anything