Ramana said, "Anything that is not present in the deep sleep state, is not real."
That has sat as a koan for me, for a long time.
I was stuck on the Vedanta conception of the three (or, four, or five, depending on what Vedanta tradition you want to look from) states of being - Waking, Dreaming & Deep Sleep which correlate to the Gross, Subtle, and Causal (or Very Subtle if you're Tibetan, or into Tibetan Buddhism) realms of existence.
I don't think that is what he was talking about, anymore.
Ramana often said that the "I-thought" created the world, which was fundamentally unreal. He also said that the true self was, "I-I". I think that, with this "I-I," he meant the I behind the "I".
The breaking of this koan for me occurs this way - The world exists. Everything I think about "it" is about it only and is not the truth of it. Thinking is mostly a repetitive sequence of concepts "stated" in words (sort of.) Words are never the thing itself. Words are simply about the thing. "Fire" cannot burn you, but fire can.
When I am in deep sleep, the world does not go away. Reality does not cease to be. That is why I can be roused by occurrences in reality. That is why I can report on how I slept the next morning. That is why the "am" portion of "I am" never leaves, or wavers, or changes.
While deeply asleep, reality still is. I still exist. It's just when I wake that the thinking, concepts, stories, and words about reality spin up. That realm of labeling, categorizing, evaluating, and judging is the delusion. That is samsara. That is the unreal. That is the suffering.
What exists in deep sleep is real. My story about it, when I wake up in the morning, is not.