I answer that, As Tully says (De Invent. Rhet. ii), suspicion denotes evil thinking based on slight indications, and this is due to three causes. 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. Consequently suspicion denotes a certain amount of vice, and the further it goes, the more vicious it is.
Now there are three degrees of suspicion. The first degree is when a man begins to doubt of another's goodness from slight indications. This is a venial and a light sin; for "it belongs to human temptation without which no man can go through this life," according to a gloss on 1 Cor. 4:5, "Judge not before the time." The second degree is when a man, from slight indications, esteems another man's wickedness as certain. This is a mortal sin, if it be about a grave matter, since it cannot be without contempt of one's neighbor. Hence the same gloss goes on to say: "If then we cannot avoid suspicions, because we are human, we must nevertheless restrain our judgment, and refrain from forming a definite and fixed opinion." The third degree is when a judge goes so far as to condemn a man on suspicion: this pertains directly to injustice, and consequently is a mortal sin.