成唯識論演祕 4.711

4.711

此六塵等有情耽染而無義利。或名六衰，能衰有情所有善法。

These refer to the six sense objects and so forth, in which sentient beings indulge with craving and which bring no benefit. Alternatively, they are called the six declines, because they are capable of causing the decline of all wholesome dharmas of sentient beings.