In things pertaining to enthusiasm, no man is sane who does not know how to be insane on proper occasions.

Related:

• Homer: This isn't fair! How can you tell who's sane and who's insane?
  Doctor: Well, we have a very simple method. ...
• In an insane society,
  the sane man must appear insane. --...
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• It is not the critic who counts; not the man who points out how the strong man stumbles,
  or where the doer of deeds could have done them better...
• You can bring men from other parts of the world who are sane.
  And you know what happens? At the very moment they...
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• The following ties in nicely with another thread (What is music?) All I know about music is that not many people ever really hear it.
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• He who studies books alone will know how things ought to be;
  and he who studies men will know how they are...