It is shown that the problem of the existence of a time function or of a semi-time function in general relativity is mathematically equivalent to that of the existence of a continuous utility function for an agent in microeconomics. Some theorems developed in economics can therefore be used in general relativity. With this strategy it is possible to give a new proof that stable causality is equivalent to the existence of a time function and also to clarify in which circumstances the causal structure can be recovered from the set of time functions allowed by the spacetime.