Putting aside everything that could not be the core or ultimate subject of experiencing you are driven to the view that there can be but one such, and that all experiencing belong to the same experiencer, unbounded by time or space or logic that pertains to objects. This can be the most terrifying of thoughts, and indeed so can any thought that points to a solitary ultimate truth of being. The error of course is in the thought, in the thinkability, but consider what expansion of understanding would be required to reverse this evaluation.