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The relation between what we see and what we know is never settled.
Mathematics is not a deductive science—that's a cliché. When you try to prove a theorem, you don't just list the hypotheses and then start to reason. What you do is trial and error, experimentation, guesswork.
的确,人们中间流行着一种奇怪的观点,即房屋、山脉、河流,总之,所有感知对象,都具有超越其被理解感知之外的自然或真实存在。