Philo Judaeus De Migratione Abrahami 222

[222] So that never, O my mind, do thou become effeminate and yield; but even if any thing does appear difficult to be discovered by contemplation, still opening the seeing faculties that are in thyself, look inwards and investigate existing things more accurately, and never close thy eyes whether intentionally or unintentionally; for sleep is a blind thing as wakefulness is a sharp-sighted thing. And it is well to be content if by assiduity in investigation it is granted to thee to arrive at a correct conception of the objects of thy search.