To hold that consciousness is an intrinsic attribute of a substance is to deny that consciousness is essentially relational, and thus to deny that it is essentially an awareness of an object outside consciousness. (p. 9)
So, who's a realist (we perceive independently existing objects), who out there is a representationalist (we can only know objects indirectly, through ideas), and who's an outright idealist (only ideas exist, there's no external world)? The above view-- of consciousness as a substance fundamentally independent of reality-- is that of Descartes, a representationalist. You see the mind-body problem in there, the insuperable dualist incompatibility.
BTW, realism does not entail that there are not specific forms of consciousness: just because I see green leaves and my dog sees them as grey does not in itself mean that we are not both perceiving the leaf rather than a representation of a leaf.
I.e., do illusions, etc. mean that we don't actually see objects? Kelley says that the relation of consciousness to existence is a deeper issue than historical figures in philosophy have usually addressed, such that the treatment of perception's various quirks is determined by preexisting premises about whether consciousness is essentially creative or passive.
This was posted earlier at my Live Journal, where there's a long discussion to clarify what I mean by "an awareness of an object outside consciousness." In short, the clarification is that the thesis of the primacy of existence is not saying you can't have an awareness of an awareness, but that any object of awareness has a pre-existing identity and, as the object of a particular act of awareness, is not identical to that awareness. An implication of this is the hierarchical nature of self-awareness-- and self-awareness is the closest actual experience of consciousness I can think of to some imagined literal interpretation of "a consciousness conscious of nothing but itself." I guess I think that phrasing lacks the necessary clarification that only the identicality interpretation is what can be considered "a contradiction in terms" (Rand, in Galt's Speech, in Atlas Shrugged)-- not that conscious phenomena (pain, memories, feelings, and much more) cannot be the sole objects being considered in focus at a particular time.
*Topic only lightly grazed.