“ for when a man hates or despises another, or is angry with or envious of him, he is led by slight indications to think evil of him, because everyone easily believes what he desires. ”
 Thomas Aquinas, Summa Theologica (1274). copy citation

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“First, from a man being evil in himself, and from this very fact, as though conscious of his own wickedness, he is prone to think evil of others, according to Eccles. 10:3, "The fool when he walketh in the way, whereas he himself is a fool, esteemeth all men fools." Secondly, this is due to a man being ill-disposed towards another: for when a man hates or despises another, or is angry with or envious of him, he is led by slight indications to think evil of him, because everyone easily believes what he desires. Thirdly, this is due to long experience: wherefore the Philosopher says (Rhet. ii, 13) that "old people are very suspicious, for they have often experienced the faults of others." The first two causes of suspicion evidently connote perversity of the affections, while the third diminishes the nature of suspicion, in as much as experience leads to certainty which is contrary to the nature of suspicion.” source