Misericordia est amor quatenus hominem ita afficit ut ex bono alterius gaudeat et contra ut ex alterius malo contristetur.
Compassion is love insofar as it affects a person in such a way that one rejoices from the good fortune of another and is saddened by the misfortune of another.
EXPLICATIO: Cæterum de invidia vide scholium propositionis 24 et scholium propositionis 32 hujus. Atque hi affectus lætitiæ et tristitiæ sunt quos idea rei externæ comitatur tanquam causa per se vel per accidens. Hinc ad alios transeo quos idea rei internæ comitatur tanquam causa.
For the rest on envy, see IIIP24S and IIIP32S. And these affects of joy and sadness are those that the idea of an external thing accompanies as if the cause of it either through itself or by accident. From here I pass to others which the idea of an internal thing accompanies as if the cause.
This distinction of an external cause versus an internal cause seems to be Spinoza's version of Reinhold Niebuhr's famous Serenity Prayer which distinguishes between what can be controlled and what cannot. In this work designed to enhance personal joy, Spinoza appears to find that affects can first be identified by their apparent cause - external or internal.
