Personally I think no-one is conscious, and there's no such thing as consciousness, and it's all some ghastly philosopher
s' bungle begun by Locke
; but if I wanted to argue about all that at length I'd be over in consciousness
where all the harangues are. For now I just want to make some linguistic points about how the words came to mean what they do.
If you know any French you might know that their word for consciousness is conscience, which is also their word for conscience; and this might have puzzled you. If you know a little of Latin you'll know the common elements are con- 'with' and sci- 'know'. How does 'with-knowledge' bear on the modern meanings of conscience and the conscious?
The OED gives us the shades of sense as they came into English, by date. Originally, in Latin, it meant knowledge with someone. Caesar knew something, and Brutus knew it too: Brutus knew it with him. This sense lingered in English for a while, longer in poetry: if you went out alone into an isolated grove and told the trees your woes, the trees knew it with you: they were conscious.
The next shift was from others as literal co-knowers to yourself as metaphorical co-knower. You could observe something in yourself, have knowledge of yourself. (This seems to have happened within Latin, and English also borrowed this sense. The word conscience is oldest, with conscious appearing from 1601, and consciousness from 1632.)
The moral sense seems more specific: you can look into yourself and know that something is so by satisfying yourself that it is. It is thus a kind of conviction, a certitude. We talk of a consciousness of a life well spent. When this self-examination fails, or reveals to us our error or weakness, we may be conscious of our guilt. This is the kind of thing this family of words meant in the early 1600s: aware by looking into our heart or mind. It was a state you might be in or not be in, like being convinced of something, or being aware of something.
The OED gives a date of 1678 and a quotation from Locke for the first explicitly philosophical sense: he said 'Consciousness is the perception of what passes in a Man's own mind.' Well that's just what I said up above, but the difference is that Locke had a specific empiricist theory of ideas and impressions, and what he meant was not that from time to time you might look into your heart, but that all the time we are perceiving what we are thinking: that we couldn't be thinking it unless the thought were passing for review in front of us, like an actor on a stage, or something of that kind. I don't want to study what exactly Locke meant, but I'm fairly clear he was the first to make the claim that consciousness is permanent, never switched off, and applied in a blanket fashion to all our thought.
Let's move from etymology to practical lexicography.
When we're not doing philosophy or some rarefied form of psychology, when do we actually use the words 'conscious(ness)'? First, by itself it means 'awake', and not just awake after a normal night's sleep, but awake after some special deprivation, like anaesthetic or drunkenness. The old Shorter OED I've got at my elbow doesn't even mention this sense, so relying on my memory of researching this in the full thing several years ago, I believe this usage is only nineteenth-century. That is, the most common ordinary use of 'conscious', absolutely, is a Johnny-come-lately of a meaning, unconnected with the classic philosophical puzzler we're all supposed to be endowed with. So if you're asking yourself 'what is consciousness?' in a metaphysical tone of voice, and you find you need to explain about being awake, you can drop that packet -- it's not in the original riddle.
'Absolutely', I said just then. That means not followed by a complement. The other common ordinary use is when it's followed by 'of' and a complement: conscious of... what? Conscious of a commotion in the street outside, conscious of not doing one's best, conscious of having fudged the issue, conscious of a spider crawling across the wall as you read. These all imply less than a full degree of concentration. You know something is there, you're not unaware of it, but you're not giving it attention. Either it's peripheral or it's something you have been blocking out or it's something you would prefer to block out. If you're reading a book you wouldn't normally say you're conscious of reading the book: you might be conscious of the unusual typeface, but not of the main thing you're focusing on.
And this sort of consciousness-of is a come-and-go thing, liable to flicker and fade and switch and lose interest and drop below the threshold. It's not a powerful spotlight from which no cranny of the mind escapes.
Why bring up the uses of the words in ordinary language? Because those are what we're certain about. I'm certain I'm aware of some things some of the time, others at others, that I'm awake and not asleep, and that I can if I choose focus my awareness on my own thoughts. I know equally well that a lot of the time I don't do any of those things, just as I don't walk or decide or frown all the time. These are the real things. Nowhere in any of this do I see any reason to believe in the philosopher's fiction invented by Locke, the dazzling illumination that also includes all the disparate ordinary senses, and is supposed to be so different from all the things we could explain. Someone who claims I'm certainly conscious is mischievously mixing things up... But let's not drift into that harangue.