Within someone who lacks knowledge and understanding, there are correct opinions about matters of which they have no knowledge or understanding.

These true opinions are stirred up and awakened within oneself as though arising from a dream.

Plato, The Meno

Theætetus: In truth, and without doubt, I cannot deny that those whose minds are impaired, or those who are dreaming, hold not false opinions - whether the former believe themselves to be Gods, or those in sleep think they are winged and fly.

Socrates: Are you or are you not aware of uncertainties concerning these matters, especially perceptions when dreaming and when awake?

Theætetus: What uncertainties?

Socrates: Ones that I believe you have often heard being asked. At this moment in time, ‘how can anyone demonstrate whether we are asleep and all our thoughts that come to mind are dreams, or whether we are watchful and, in reality, conversing?’

Theætetus: Surely, O Socrates, it is doubtful that there can be any proof for this. For these things behave, as it were, in the same way as each other. For, with respect to our present conversation, nothing prevents us from believing that we are in a dream, and also continuing on this conversation here – and when asleep, we form images brought to us by our dreams, the likeness to our conversation is remarkable.

Socrates: You see, then, it is not difficult to dispute this matter since it is doubtful whether we are watchful or we are looking at things in our dreams. Especially since the time we spend sleeping is equal to the time we spend being watchful – and in each state, our soul asserts that the images are as they appear so that for the amount of time we say these things are true, we say for the equal amount of time that those other things are true and we strenuously profess their reality in each case.

Plato, The Theætetus

We frequently acquire knowledge of future events through dreams when we are asleep and not being in a tumultuous ecstasy at that time because the body is at rest. But we do not understand what is happening in the same manner as when we are awake.

Iamblichus, On The Mysteries of the Egyptians, Chaldeans, and Assyrians