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Knowledge of other minds is possible but cannot be gained by attention to behavior.
I say the similitude of passions, which are the same in all men,- desire, fear, hope, etc.; not the similitude of the objects of the passions, which are the things desired, feared, hoped, etc.: for these the constitution individual, and particular education, do so vary, and they are so easy to be kept from our knowledge, that the characters of man's heart, blotted and confounded as they are with dissembling, lying, counterfeiting, and erroneous doctrines, are legible only to him that searcheth hearts. And though by men's actions we do discover their design sometimes; yet to do it without comparing them with our own, and distinguishing all circumstances by which the case may come to be altered, is to decipher without a key, and be for the most part deceived, by too much trust or by too much diffidence, as he that reads is himself a good or evil man.
But let one man read another by his actions never so perfectly, it serves him only with his acquaintance, which are but few.
Hobbes, Lev. Intro. 3-4