Perception

Reality is not out there, apart from our mind. Our mind is part of reality. And what we observe as reality is in some sense an illusion, because it is subjectively interpreted by us. We are like reality observing itself, but only one interpretation of reality. 

Our perception is not all that there is. Our perception is only a tiny fraction of all that there is. Any interpretation we might have of reality can only be partial. We are prone to error and there is a barrier between our subjective experience of reality and the nature of reality itself. 

To some extent, we can make our own reality. But there is also an objective reality which imposes boundaries on what we can do. We have a hard time judging where those boundaries are. Science, mathematics, logic, and history give us an idea, but not an absolute one. This universe and the human mind are deeply complex and mysterious.

The greater truths tend to be counter-intuitive and tend to contradict the status quo. The truth can be very uncomfortable and inconvenient. It can burst a lot of bubbles and threaten egos. Humanity has not finished pushing the boundaries of what we know about ourselves and our universe. Many more bubbles will be burst. Many more egos will be bruised.

P.S. For more in-depth thinking and research on this, check out Biocentrism and Incognito.