Theories
Definition
So much is written on the subject (consciousness) based on so many different definitions that we first need to make explicit what we mean by consciousness.
Together with Gray and Velmans I choose (opt) for a pragmatic approach, closest to daily life. Consciousness or conscious awareness refers to the subjective experiencing of phenomenal content. It is the implicit notice of a 'whole me' that is experiencing something. 'Implicit' because we don't need to be (meta)conscious of the fact that it is me experiencing something, while at the same time consciousness has everything to do with my perspective (that of my body's, that is...) on life.
For an overview of definitions i would recommend to read Velmans and a special theme number of the Journal of Consciousness Studies about the subject.
Approaches
Theories of consciousness can be roughly divided into dualist and monist approaches.
Dualist theories are based on the ideas of Plato and later Descartes, who consider mind and matter as two different substances that interact but cannot be reduced to eachother. For an extensive review I recommend Velmans. Although dualist theories have become less popular in our time, there are still scientists advocating it. These theories generally claim dualist-interactionism, in which consciousness has causal power on the material world and vice versa. Eccles for example assumed that the self-conscious mind has non-material existence of its own, being able to influence brain processes and giving a special status to free will. According to Eccles consciousness could not have developed by natural selection if it only were an epiphenomenon. Other examples are the quantum theory of consciousness from Hameroff and Penrose (see Gray for an extensive analysis) and the theories that equal consciousness with electromagnetic field generated and fed back to brain networks (see McPhaddon).
These theories have in common that they assume that "autonomously existing experiences can have two-way interactions with the brain" (Velmans). In order for consciousness to influence brain states there should be a gap in neural causation chains to operate. As far as evidence suggests, the physical world is causally closed.
Dualist-interactionism theories have many problems, like explaining how something immaterial could influence matter without violating the Second Law of Thermodynamics (conservation of energy).Monist theories of consciousness try to reduce mind and matter to a common, underlying principle. That can be mind (idealism), matter (physicalism, functionalism) or something else (duals-aspect theory, neutral monism, pan-psychism, dynamism).
Spinoza considered mind and body being different aspects of one underlying reality that you can give any name (Nature, God). It corresponds with my concept of Cruality, which refers to 'unknowable reality'. Modern varieties are dual aspect theories of information (Velmans, Nagel, Chalmers). Neutral monism takes the stance that what we see is neither intrinsically mental or physical. We judge what we experience to be that. (zie Mach, in Velmans p.32). It corresponds with my concept of 'nested sensation'. (see my notes in Velmans)
Idealism claims that things only exist in the mind. A special version is the Copenhagen interpretation of Quantum Mechanics (Velmans, p. 37) that assumes things coming into existence only through observation (the act of observation makes the wave form collapse). However, perception is bound to observation, material processes continue on their own even when not observed (like the apple pie baking in the oven).
Physicalism is the strongest represented view in modern science. It reduces consciousness to a state of the brain. But it leaves us with a big gap between conscious experience and (measured) brain states.
