  We show how G\"odel's first incompleteness theorem has an analog in quantum
theory. G\"odel's theorem implies endless opportunities for appending axioms to
arithmetic, implicitly showing a role for an agent, namely an agent that
asserts an axiom. There is an analog of this theorem in physics, to do with the
set of explanations of given evidence. We prove that the set of explanations of
given evidence is uncountably infinite, thereby showing how contact between
theory and experiment depends on activity beyond computation and measurement --
physical activity characterized as agency.
