成唯識論述記 1.47

1.47

「樂」謂五樂：一者，因樂。即順樂受諸根境界，樂相應法，樂所發業，皆名因樂，能有樂故。

"Gladden" (or "happiness") refers to five kinds of happiness: First, causal happiness. That is, the sense faculties and sense objects that accord with pleasant feeling, the dharmas associated with pleasure, and the karma generated by pleasure—these are all called causal happiness, because they are capable of producing happiness.