The aim of this of this paper is to study infinite games and to prove formally some properties in this framework. As a consequence we show that the behavior (the madness) of people which leads to speculative crashes or escalation can be fully rational. Indeed it proceeds from the statement that resources are infinite. The reasoning is based on the concept of coinduction conceived by computer scientists to model infinite computations and used by economic agents unknowingly. When used consciously, this concept is not as simple as induction and we could paraphrase Newton: ''Modeling the madness of people is more difficult than modeling the motion of planets''.