I am a quantum physicist by training, and it is often remarked that quantum physics is incredibly hard to get one’s head around, as it is so counter-intuitive, however in my opinion the counter-intuitiveness of quantum physics pales into insignificance compared to trying to get one’s head around the concept of consciousness. There is nothing more counter-intuitive than trying to contemplate the concept that you (as a conscious entity) might not exist.
My favourite quote on consciousness comes from the “Macmillan Dictionary of Psychology” (1989 edition), Stuart Sutherland expressed a sceptical attitude more than a definition:
Consciousness — The having of perceptions, thoughts, and feelings; awareness. The term is impossible to define except in terms that are unintelligible without a grasp of what consciousness means. Many fall into the trap of equating consciousness with self-consciousness—to be conscious it is only necessary to be aware of the external world. Consciousness is a fascinating but elusive phenomenon: it is impossible to specify what it is, what it does, or why it has evolved. Nothing worth reading has been written on it.