A passage from Epictetus
Epictetus, Enchiridion 16
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When you see anyone weeping in grief because his son has gone abroad, or is dead, or because he has suffered in his affairs, be careful that the appearance may not misdirect you. Instead, distinguish within your own mind, and be prepared to say, 'It's not what has happened that distresses this man, but his judgement about it.'
Related passages
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Begin the morning by saying to thyself, I shall meet with the busy-body, the ungrateful, arrogant, deceitful, envious, unsocial.
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Take away thy opinion, and then there is taken away the complaint, 'I have been harmed.' Take away the complaint, and the harm is taken away.
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It is a man's special privilege to love even those who wrong him.