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Those are the two principles. What's just as important, however, is for us to understand how Rawls argues for the principles. In other words, why does he believe that rational parties in the original position would choose these principles? Remember, there are no independent criteria. Everything depends on what people in the original position would choose.

Rawls believes that in the original position it would be rational for people to apply what is called the "maximin" decision strategy.

The maximin rule tells us to rank alternatives by their worst possible outcomes: we are to adopt the alternative the worst outcome of which is superior to the worst outcomes of the others. [Theory, section #26]

Now, Rawls knows that it is not rational in all situations to adopt the maximin rule. But he believes that it is entirely rational when any of the following conditions apply. First, there is no way to estimate the probability of various outcomes. (Will I be rich? Will I be poor? Nobody knows.) Second, one cares very little for any gain beyond the minimum guaranteed by this rule. And third, the alternatives have unacceptable results.

Rawls has defined things so that people in the original position cannot know the probabilities of the various contingencies of real life. Furthermore, one of the leading alternatives -- the principle of utility -- would allow their welfare to be traded off to increase the welfare of others. Given this situation, he concludes that the parties to the original position would choose to apply the maximin rule; and furthermore, that the maximin rule supports his two principles. After all, not only do his principles guarantee us equal liberties, but they guarantee that we will always gain from any increase in inequality. And, perhaps most of all, everyone is guaranteed that their well-being will not be traded off in order to maximize the total sum of well-being.