An eye for an eye makes the whole world circumspect. But, also, an eye for an eye makes the whole world cooperative, as Robert Axelrod showed with his study of tit for tat and competing strategies using iterated rounds of contests among genetic algorithms (described in his book The Evolution of Cooperation). Tit for tat beat all the alternative strategies, again and again; and as rounds of the contest were iterated, with winning strategies favored by the reproductive mechanism of the iteration, it more and more perfused the population of competing algorithms. As tit for tat increased in frequency, so did the total value generated by all competitors in each round: fewer and fewer defections occurred, and responses to defections were more and more often optimal.
That tit for tat wins the evolutionary game does not mean that its superiority is merely adventitious, an artifact of this or that sequence of random events that might have been quite different, and so generated quite a different sort of winner. On the contrary: provided the game goes on long enough, tit for tat wins every time, sooner or later, and no matter how the sequence of outcomes varies. The utile superiority of tit for tat is a truth of game theory, so that like any other mathematical truth it is from before any and all worlds, and holds true in every world. The metaphysical superiority of tit for tat, then, is the source and reason of its practical evolutionary success, and not vice versa (this is true of all perdurant evolutionary success). Tit for tat is the optimal strategy in evolutionary practice because it is the best in metaphysical fact. As metaphysically best it is the most moral policy of all (these are two ways to say the same thing).