Know that "interval" is an expression for something which separates two other things, like the diving line between sun and shade, and as He said—may He be exalted—concerning the mixture of the two seas, "Between them is a barrier (barzakh) which they cannot cross" [Koran 55:20]. The meaning of "they cannot cross" is that they cannot mix with one another because of this partition which divides them. The sense of sight does not discern it. When suddenly it is perceived, the barrier does not exist. And when the barrier is between the known and the unknown, the nonexistent and the existent, the negated and the affirmed, and the rational and the irrational, it is called Interval—and [this Interval] is the imagination.
For if you perceive it—and you are rational—you know that your vision has encountered an existent thing, while you know unequivocally that it is not a "thing" completely and fundamentally. And what is this whose "thingness" is affirmed and denied simultaneously? The imagination is not existent or nonexistent, not known or unknown, not negated and not affirmed. And the human being travels to this reality in his sleep and after his death, and he sees descriptive qualities as existing embodied forms, and there is no doubt of that. And the intuitive person sees in his waking state what the sleeper sees in the state of sleep and the deceased after death.
---from Journey to the Lord of Power: A Sufi Manual on Retreat by Ibn 'Arabi.