 A more significant problem with invoking the Nash equilibrium as the appropriate solution concept arises because some games have multiple Nash equilibria (see the section on  Solution Concepts and Equilibria,  in the entry on game theory). When there are several different Nash equilibria, how is a rational agent to decide which of the several equilibria is the “right one” to settle  upon?[7]  Attempts to resolve this problem have produced a number of possible refinements to the concept of a Nash equilibrium, each refinement having some intuitive purchase. Unfortunately, so many refinements of the notion of a Nash equilibrium have been developed that, in many games which have multiple Nash equilibria, each equilibrium could be justified by some refinement present in the literature. The problem has thus shifted from choosing among multiple Nash equilibria to choosing among the various refinements.