Decision theory is enough to advise actions - so why do we need game theory? A game theory is really just a theory about the distribution over how other agents think. Given such a distribution, decision theory is enough to tell you what to do. So any simple game theory, one that claimed with certainty that all other agents always think a particular way, must be wrong. Of course sometimes a simple game theory can be good enough - if slight variations from some standard way of thinking doesn't make much difference. But when small variations can make large differences, the only safe game theory is a wide distribution over the many ways other agents might think.