Those passions which are restrained by the sense of propriety, are all in some degree moderated and subdued by it.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“But the man who, in relating to some other person the injury which has been done to him, feels at once the fury of his passion cooled and becalmed by sympathy with the more moderate sentiments of his companion, who at once adopts those more moderate sentiments, and comes to view that injury, not in the black and atrocious colours in which he had originally beheld it, but in the much milder and fairer light in which his companion naturally views it; not only restrains, but in some measure subdues his anger. The passion becomes really less than it was before, and less capable of exciting him to the violent and bloody revenge which at first, perhaps, he might have thought of inflicting.
Those passions which are restrained by the sense of propriety, are all in some degree moderated and subdued by it. But those which are restrained only by prudential considerations of any kind, are, on the contrary, frequently inflamed by the restraint, and sometimes (long after the provocation given, and when nobody is thinking about it) burst out absurdly and unexpectedly, and with tenfold fury and violence.
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