  How do we ascribe subjective probability? In decision theory, this question
is often addressed by representation theorems, going back to Ramsey (1926),
which tell us how to define or measure subjective probability by observable
preferences. However, standard representation theorems make strong rationality
assumptions, in particular expected utility maximization. How do we ascribe
subjective probability to agents which do not satisfy these strong rationality
assumptions? I present a representation theorem with weak rationality
assumptions which can be used to define or measure subjective probability for
partly irrational agents.
