St. Augustine, Letters
38.2 (to Profuturus; tr. James Houston Baxter):
Although no angry person thinks his own anger is unjustified, it grows upon him, and anger that becomes inveterate in this way passes into hatred, since the pleasureableness that accompanies an apparently justified resentment keeps it longer in the vessel until the whole thing grows sour and spoils the vessel. For this reason it is much better to be angry with no one, even when it is justifiable, than from apparently justified anger to slip by the stealthy tendency of passion into hatred of anyone. We have a proverbial saying about welcoming unknown guests that it is much better to endure a bad man than through ignorance to risk shutting out a good one from fear of welcoming a bad one. But with our passions the opposite is true: for it is beyond comparison a more beneficial thing not to open the shrine of our heart at the knock of even justified anger than to yield it entrance: once in, it will not easily be expelled, and it will grow from a sapling to a sturdy tree, since it boldly and shamelessly develops at an even greater speed than people imagine, for it is not put to shame in the darkness, when the sun has gone down upon it.
subrepit autem, dum nulli irascenti ira sua videtur iniusta. ita enim inveterescens ira fit odium, dum quasi iusti doloris admixta dulcedo diutius eam in vase detinet, donec totum acescat vasque corrumpat. quapropter multo melius nec iuste cuiquam irascimur, quam velut iuste irascendo in alicuius odium irae occulta facilitate delabimur. in recipiendis enim hospitibus ignotis solemus dicere multo esse melius malum hominem perpeti quam forsitan per ignorantiam excludi bonum, dum cavemus, ne recipiatur malus. sed in affectibus animi contra est. nam incomparabiliter salubrius est irae etiam iuste pulsanti non aperire penetrale cordis quam admittere non facile recessuram et perventuram de surculo ad trabem. audet quippe inpudenter etiam crescere citius, quam putatur. non enim erubescit in tenebris, cum super eam sol occiderit.