What is that evidence? Doesn't that depend upon how one defines consciousness? whose definition of consciousness should we be using and why?
The evidence is that when the brain stops behaving in certain ways, consciousness ceases – both from a first and third party perspective. When you fall into a dreamless sleep, or when you get knocked out, the brain stops functioning in a way which sustains consciousness, and consciousness only resumes when the brain resumes its normal functioning.
Regarding the definition of consciousness, well that’s the core of the whole problem isn’t it? We can talk about the features of consciousness – e.g. sensory information, memory, thought processing - without too much trouble but when we get to awareness itself we run into all kinds of difficulty, because awareness is something which only seems to exist when at least one of the aforementioned features is also present. A couple of thought experiments might be useful here:
1)Try to think of any memory, thought, emotion, or other mental process which doesn’t originate with some sort of external sensory data. That external data comes in the form of light, sound, or electro-chemical messages, all of which are material phenomena.
2)Take a conscious person, and one-by-one remove the features of their consciousness. First remove all external sensory input, then all thinking and imagining, then all emotional impressions, then long-term memory, then finally short-term memory. After all these have been removed, what remains? Nothing, as far as conscious mental activity is concerned. With nothing to be conscious of, awareness itself falls away.
Bring all these features back online, and what you notice is that what we call consciousness/awareness is not really a “-ness” but a continual feedback
process which roughly goes something like this:
Sense data + memory capability + thought and emotion processing capability -> creation of memory of sense data -> new sense data -> formation of new memory -> cross referencing with previous memory -> generation of emotional and thought impressions -> creation of thought/emotion memory -> (rapid ad nauseum repetition of previous steps) -> development of a mental construct of a “self” - > self-awareness -> initial sequence of steps filtered through the mental construct of a “self” -> more compelling sense of self-awareness-> impression that consciousness is a real, substantial independent thing.
Keep in mind that all of this happens extremely rapidly and begins (presumably) in the womb. So like a roll of film moving at high speed through a projector, there seems to be a moving stream of consciousness, and the thought processes have a whole lifetime to reinforce the misleading impression.