If consciousness is a scientific fact, science should be able to discover what causes it. The scientific approach to a fact is to explain it, not to ignore or dismiss it, so the question raised is:
“Why does conscious experience exist?” (Chalmers, 1996), (p5).
A major review of theories on consciousness divides them exhaustively into the following categories labelled A-F (Chalmers, 2003):
A. Materialism-A. Consciousness doesn’t exist, except as imagined by the physical brain (Dennett, 1991). Physical causes explain everything but the physical brain just invents an observer, so the hard problem doesn’t exist.
B. Materialism-B. Consciousness does exist, but is identical to certain physical brain states for all practical purposes (Block & Stalnaker, 1999). Again if physical causes explain everything, consciousness must equate to a brain state, so the hard problem is solved.
C. Materialism-C. Consciousness does exist, but is in theory a physical brain effect (Nagel, 1974) (Edelman, 2003). That physical causes explain everything then just means they will one day explain consciousness, so the hard problem will be solved eventually.
Materialism then concludes that either consciousness doesn’t exist (A), or if it does, it is just a physical brain state (B), or some other physical cause that the future will find (C). Others however suggest various non-physical causes, including:
D. Dualism-D. Consciousness exists by itself in a non-physical realm, where it affects matter which in turn also affects it (Stapp, 1993). Consciousness is then independent of matter, but interacts with it, so the hard problem is solved.
E. Dualism-E. Consciousness is a brain by-product that helps us survive but doesn’t affect matter (Zizzi, 2003). Consciousness is then an epiphenomenon, like a train whistle that exists but doesn’t affect the physical train, so the hard problem is solved.
F. Neutral Monism-F. Both consciousness and matter derive from a primal cause that is neither, as a common ancestor causes both (Russell, 2005). Consciousness and matter then both exist but are generated, and neither causes the other, so the hard problem is solved.
In these theories, consciousness exists as matter does in another realm (D), or it exists there but unlike matter affects nothing (E), or something else entirely causes both it and matter (F).
The proposed causes of consciousness then divide into those that attribute it to physical states and those that imply a non-physical cause. How science assesses these theories is now reviewed, starting with physical causes.