But how does he know which principl

But how does he know which principles are especially in his interests? The same holds for the formation of coalitions: if a group were to decide to band together to the other,
they would not know how to favor themselves in the choice of principles.
Even if they could get everyone to agree to their proposal,
they would have no assurance that it was to their advantage,
since they cannot identify them-assurance that it was to their advantage,
since they cannot identify them-selves either by name or description.
The one case where this conclusion fails is that of saving.
Since the persons in the original position know that they are contemporaries (taking the present time of entry interpretation),
they can favor their generation by refusing to make any sacrifices at all for their successors; they simply acknowledge the principle that no one has a duty to save for posterity.
Previous generations have saved or they have not; there is nothing the parties can now do to affect that.
So in this instance the veil of ignorance fails to secure the desired result. Therefore,
to handle the question of justice between generations,
I modify the motivation assumption and add a further constraint ($22). With these adjustmemt,
no generation is able to formulate principles especially designed to advance its own cause and some significant limits on savings principles can be derived ($44).
Whatever a person’s temporal position, each is forced to choose for all.