Based on the observation of mere cognisance, the non-observation of [outer] referents arises. Based on the non-observation of referents, also the non-observation of mere cognisance arises. Thus, one engages in the characteristic of the nonexistence of both apprehender and apprehended. Therefore, observation is established as the nature of non-observation, because if there is no referent to be observed, an observation [of it] is not suitable. Thus, one should understand observation and non-observation as being equal. 

-- Vasubandhu