On Not Wanting to Know

A common assumption of practical reasoning is that, in order to act rationally, agents are to act on the basis of the totality of evidence available to them. Common practice and introspection, however, suggest that people often do not want to know. The chapter explores various aspects of the phenomenon of not wanting to know in an attempt to find out whether it is inherently unreasonable. The exploration leads, first, to weakening the principle of total evidence through replacing it with a rebuttable presumption in favor of additional knowledge. The sustainability of this presumption is then examined in light of the large variety of circumstances in which it seems to be reasonably rebutted. The alternative which in the end is recommended is to give up both the general principle and the presumption, and adopt instead something like a case by case cost-benefit approach, where the value of additional knowledge is matched up against its cost.