Cognitively, so is our construction of what we consider reality. It’s likely real, I don’t harbor any doubts, but consider how we perceive experience:
It takes approximately 400ms to consciously register a change from expectations and we consider the “Just happened in the recent past” as “THE PRESENT”.
Also, certainty is but an emotion, like uncertainty. Nothing special about it. All fantasy. What corresponds to other people’s experiences or reality? Well, that’s the closest we can to to something that we can imagine is “not fantasy”.
But it still is. It’s just a fantasy that happens to correspond with tangible things.