Our first objective in undertaking the experimental program described here has been to gain some understanding of what goes on in long sequences of plays of Prisoner's Dilemma. We have attempted to gain this understanding by postulating a system going through a sequence of states and by attempting to formulate some mathematical models from which the dynamics of the system could be deduced. Once such a model is found, its parameters, properly interpreted, become the key terms in the emerging psychological theory. This strategy can be deemed successful, if the parameters so discovered are independent of the process itself, if they suggest further investigation, and if the further investigations, in turn, lead to a more inclusive theory.

p. 185 - Prisoner's dilemma: A study in conflict and cooperation (1965)