大般涅槃經義記 5.119

5.119

三對緣分別，緣有三差：謂違、順、中。違緣逼惱名為苦受，順緣生適名為樂受，與二相違名不苦不樂。

Third, distinguishing in terms of conditions: conditions have three differences, namely contrary, accordant, and neutral. A contrary condition that oppresses and afflicts is called the feeling of suffering; an accordant condition that generates delight is called the feeling of pleasure; what is contrary to both is called neither-suffering-nor-pleasure.