I contributed; Ulam did not. I'm sorry I had to answer it in this abrupt way. Ulam was rightly dissatisfied with an old approach. He came to me with … - Edward Teller

When you come to the end of all the light you know, and it's time to step into the darkness of the unknown, faith is knowing that one of two things shall happen: either you will be given something solid to stand on or you will be taught to fly.

I hate doubt, yet I am certain that doubt is the only way to approach anything worth believing in.

The preservation of peace and the improvement of the lot of all people require us to have faith in the rationality of humans. If we have this faith and if we pursue understanding, we have not the promise but at least the possibility of success. We should not be misled by promises. Humanity in all its history has repeatedly escaped disaster by a hair's breadth. Total security has never been available to anyone. To expect it is unrealistic; to imagine that it can exist is to invite disaster. What we do have in our technological capacities is an opportunity to use our inventiveness, our creativity, our wisdom and our understanding of our fellow beings to create a future world that is a little better than the one in which we live today.