Derek Allen said: 'Part of the problem in all this springs, I often think, from the ambiguity of the word 'conscious' and the different ways we use the word. If I say 'I was not conscious of his presence', the word has a different shade of meaning from 'I was not conscious because I was knocked out' and that again has a different shade of meaning from 'I am a conscious human being” (i.e. whether I like it or not).'

Why not replace the word "consciousness" with the word "experience"?  I can then say that certain empirical descriptions of experience agrees with my experience. Can humans exist and interact without an experience that is like my experience? They probably can, after all, I exist after a bang on the head than makes me unconscious, without experience, yet may still have a tendon reflex.  Perhaps some types of serious brain damage would allow humans to exist with the lack of experience that I call "unconsciousness" yet still be able to react to stimuli (some types of PVS, akinetic mutism etc).  However, as far as I know, a case has never been found of a fully functioning human being without any experience.  The empirical evidence is that the organic machine we call "human beings" requires an experience like the empirical descriptions I discussed in an earlier comment to function optimally.  The zombie argument can then be seen to get us nowhere except a debate about degrees of function (which I think DA was also suggesting).

Returning to the definition of "consciousness", I believe the vagueness of terminology suits both those who maintain that there is no special phenomenon and those who declare the opposite.  As an example, DA is using the vagueness of definitions to diminish the idea of "phenomenal consciousness" without actually tackling the issue of how we could have our experience.