"I've been a professor of mathematics at Harvard and at Yale. At Yale for a long time. But I'm not a mathematician only. I'm a professor of physics, of economics, a long list. Each element of this list is normal. The combination of these elements is very rare at best."

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If you ask which of the scenarios I think is most dangerous, though, I will give a different answer. In that form of the question, I regard a nuclear attack, terrorist-generated or otherwise, as the most threatening combination of likelihood and long-term damage to modern life today.

— John L. Casti