Irrisio est lætitia orta ex eo quod aliquid quod contemnimus in re quam odimus inesse imaginamur.
Mockery is a joy arising from where we imagine something which we scorn to be in something which we hate.
EXPLICATIO: Quatenus rem quam odimus contemnimus eatenus de eadem existentiam negamus (vide scholium propositionis 52 hujus) et eatenus (per propositionem 20 hujus) lætamur. Sed quoniam supponimus hominem id quod irridet odio tamen habere, sequitur hanc lætitiam solidam non esse. Vide scholium propositionis 47 hujus.
Insofar as we scorn that which we hate to such a degree do we deny the existence of a thing (see IIIP52S) and to such a degree (by IIIP20) do we rejoice. But since we suppose a person has hatred for that which one mocks, it follows that this joy is not solid. See IIIP47S.
Spinoza is highlighting the complexity of moods around memories. In order to attain blessedness, clarity or processing of past events is important. For that reason, he revisits the issues raised by the things hated in the past.