It often takes pains to allow us to see all that our friends know, while they have still the advantage of not knowing to the full when we have penetrated the depth of the heart.
 François de La Rochefoucauld, Reflections; or Sentences and Moral Maxims (1665). copy citation

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Author François de La Rochefoucauld
Source Reflections; or Sentences and Moral Maxims
Topic pain depth
Date 1665
Language English
Reference
Note Translated by J. W. Willis Bund
Weblink http://www.gutenberg.org/files/9105/9105-h/9105-h.htm

Context

“We should talk of subjects that concern them, but only so far as they like, and we should take great care where we draw the line. There is a species of politeness, and we may say a similar species of humanity, which does not enter too quickly into the recesses of the heart. It often takes pains to allow us to see all that our friends know, while they have still the advantage of not knowing to the full when we have penetrated the depth of the heart. Thus the intercourse between gentlemen at once gives them familiarity and furnishes them with an infinite number of subjects on which to talk freely. Few persons have sufficient tact and good sense fairly to appreciate many matters that are essential to maintain society.” source