"And you my friend, how do you know I am not a fish — you are
not I?" ;) It's splitting hairs... Yes of course there is no absolutely precise definition of human consciousness, but it does not mean that the term becomes useless. I can't draw a precise line between a plain and a mountain (where does one start and the other end?), but it does not mean I can never tell about anything that it's not a mountain
The term doesn't become useless, but it becomes non-usable where laws and morality are concerned. Should we experiment on cognitively impaired people because they are less intelligent than a dog or a dolphin? How about month-old babies?
Or in another way. The fact that we do not differentiate (in terms of "life value") between mentally handicapped, or otherwise helpless humans and the rest, is a result of a separate ethical choice that regulates relations between us as human individuals. This choice does not logically require accepting the notion of any animal rights.
Intelligence and consciousness are not synonymous at all. As for the month-old babies (or a boxer knocked unconscious - not as cute, but still ;) ), I think consciousness in this context should be understood as something specific of our species as such, rather than of any individual in particular.
