Tristitia potest esse delectationis causa, quia tristitia siquidem in actu existens est causa delectationis, inquantum facit memoriam rei dilectae, de cuius absentia aliquis tristatur, et tamen de sola eius apprehensione delectatur.
Sadness can cause delight because sadness, as actually existing, causes delight, inasmuch as it brings to mind that which is loved, the absence of which causes sadness; and yet the mere thought of it gives delight.
Memoria autem tristitiae fit causa delectationis, propter subsequentem evasionem. Nam carere malo accipitur in ratione boni; unde secundum quod homo apprehendit se evasisse ab aliquibus tristibus et dolorosis, accrescit ei gaudii materia.
The recollection of sadness becomes a cause of delight, on account of the deliverance which ensued, because absence of evil is looked upon under the formal aspect of good; wherefore so far as a man thinks that he has been delivered from that which caused him sorrow and pain, so much reason has he to rejoice.
Augustinus dicit in VIII Confess. dicit quod "quanto maius fuit periculum in proelio, tanto maius erit gaudium in triumpho".
Augustine (Confess. viii, 3) says that "the more peril there was in the battle, so much the more joy will there be in the triumph."