What is consciousness?
Consciousness is owned boundary repair, read from the inside. Every observer holds a boundary with its neighbors and keeps records, and a thermostat does that much without a mind. Some observers also keep readings of their own state and act on them. In an experiencing system, redness and pain are organized self-reading of this kind. Owned, because the effort comes out of the observer's own budget and the repair belongs to the patch that has to update from it. Read from the inside, because the reading is the whole of having it. A complete report of your state can tell another observer exactly what you feel without the feeling becoming theirs, since having it means being the observer whose repair it drives. A molecule-for-molecule duplicate would feel the same, and that feeling would be the duplicate's own, a separate experience from yours. Which brain cells carry which distinction is a question for experiment.
Also in Appendix A: Eighteen Questions.
This answer comes from OPH, Observer Patch Holography. OPH is a rigorous, consistent and complete description of the unified, self-creating computational Universe we inhabit. Its consistency is machine-checked: the fixed point at its core is constructed explicitly, and a theory with a model contains no contradiction.
To understand OPH and check every step yourself, start with the free book Reverse Engineering Reality. It builds the theory from the ground up, and the OPH Notebook answers your questions while you read.
read the free bookask the notebookor ask sage:read chapter 40: Does It Feel Like Anything, and What Is Left to Decide?watch a universe make its own particles