From one viewpoint there were (potentially) a multiplicity of Gleisters; but from another viewpoint there was only one, and he was that one. After al… - Robert Sheckley

I poured Franklin another cup of coffee and he looked at me, his big eyes pleading. The deadheads always look like that when we reach this point. They think that Mars is like Alaska in the ’70s, or Antarctica in 2000; a frontier for brave, determined men. But Mars isn’t a frontier. It’s a dead end.

We have propounded our beliefs in various ways, and according to various doctrines. Often we have aroused the passions of men to murder and war. This was perfectly proper, since it brought the problems of morality and religion to their highest and most exquisite pitch, and gave many complicated matters for us theologians to talk about.
We argued always, and we published our various dissenting opinions. But we argued like lawyers in a court, and nobody in his right mind listens to a lawyer. Those were the days of our pride, and we never noticed that men had ceased to pay attention to us.

Not only can you not step into the same river twice; it is not even the same you who can’t step into the same river twice. Everything modifies everything. There is no niche in the past waiting for him to come back and occupy it. Nature will tolerate a paradox, but she abhors a vacuum.