Sextus Empiricus Pyrrhoniae Hypotyposes 2.1.10

[10] If, however, they say that it is not this kind of apprehension that ought, in their view, to precede inquiry, but simply mental conception, then it is no longer impossible for those who suspend judgement to inquire about the reality of things non-evident. For the Sceptic is not, I suppose, prohibited from mental conception which arises through the reason itself as a result of passive impressions and clear appearances and does not at all involve the reality of the objects conceived; for we conceive, as they say, not only of real things but also of unreal. Hence both while inquiring and while conceiving the Suspensive person continues in the Sceptical state of mind. For, as has been shown, he assents to what he experiences by way of subjective impression, according as that impression appears to him.