Laetitia et tristitia sunt contraria quia cum obiecta delectationis et tristitiae, seu doloris, sint contraria, scilicet bonum praesens et malum praesens, sequitur quod dolor et delectatio sint contraria.
Joy and sadness are contrary to one another because since the objects of pleasure and sadness (i.e., pain), viz. present good and present evil, are contrary to one another, it follows that pain and pleasure are contrary to one another.
Sicut philosophus dicit X Metaphys., contrarietas est differentia secundum formam. Forma autem, seu species, passionis et motus sumitur ex obiecto vel termino.
As the Philosopher says (Metaph. x, 4), contrariety is a difference in respect of a form. But the form (i.e., species) of a passion or movement is taken from the object or term.
Dolor ipse potest esse delectabilis per accidens, inquantum scilicet habet adiunctam admirationem, ut in spectaculis; vel inquantum facit recordationem rei amatae, et facit percipere amorem eius, de cuius absentia doletur. Unde, cum amor sit delectabilis, et dolor et omnia quae ex amore consequuntur, inquantum in eis sentitur amor, sunt delectabilia. Et propter hoc etiam dolores in spectaculis possunt esse delectabiles: inquantum in eis sentitur aliquis amor conceptus ad illos qui in spectaculis commemorantur.
Pain itself can be pleasurable accidentally, insofar as it is accompanied by wonder, as in stage-plays; or insofar as it recalls a beloved object to one's memory, and makes one feel one's love for the thing, whose absence gives us pain. Consequently, since love is pleasant, both pain and whatever else results from love, inasmuch as they remind us of our love, are pleasant. And, for this reason, we derive pleasure even from pains depicted on the stage: insofar as, in witnessing them, we perceive ourselves to conceive a certain love for those who are there represented.
Voluntas et ratio supra suos actus reflectuntur, inquantum ipsi actus voluntatis et rationis accipiuntur sub ratione boni vel mali. Et hoc modo tristitia potest esse materia delectationis, vel e converso, non per se, sed per accidens: inquantum scilicet utrumque accipitur in ratione boni vel mali.
The will and the reason reflect on their own acts, inasmuch as the acts themselves of the will and reason are considered under the formal aspect of good or evil. In this way, sadness can be the matter of pleasure, or vice versa, not essentially, but accidentally: that is, insofar as either of them is considered in the formal aspect of good or evil.
Augustinus dicit, XIV de Civ. Dei, quod "laetitia est voluntas in eorum consensione quae volumus, tristitia autem est voluntas in dissensione ab his quae nolumus".
Augustine says (De Civ. Dei xiv, 6) that "joy is the will consenting to the things we will, and sadness is the will dissenting from the things we do not will".