"I think that space flight is a condition of Nature that comes into effect when an intelligent species reaches the saturation point of its planetary habitat combined with a certain level of technological ability... I think it is a built-in gene-directed drive for the spreading of the species and its continuation."

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Everything move...you wonder how it all knows where to go. Einstein wondered how birds knew where to migrate to. He thought they might follow lines of light in the sky. He saw everything as lines of light. That's how he was built. So we don't know how he moved, either. Any more than the birds.

— Geoff Ryman