Mad people don't know they are mad. I know I am, therefore I am not.

"Begin with a function of arbitrary complexity. Feed it values, "sense data". Then, take your result, square it, and feed it back into your original function, adding a new set of sense data. Continue to feed your results back into the original function ad infinitum. What do you have? The fundamental principle of human consciousness."

In essence, we change the world by reacting to it. We learn about the world, we act on that knowledge, and we change the world, thus changing our knowledge of it and our following actions. But what if there is no consciousness, only a vessel for it? Where does consciousness even begin, what is it?

In medicine, a person is called braindead if there is no reaction from the brainstem. That is to say spinal reflexes may work fine but regulatory controls (heat, CO2 levels) are nonresponsive, as is the person in general. Yet it has been shown that people in vegetative state sometimes can react to outside stimuli, just not with a visible reaction - the response can only be seen on an MRI. Sometimes people recover from vegetative states after being in it for years. Even modern technology has difficulty laying down the line from where there is no entity that could be called consciousness, there is no soul. Then again, medicine has never been an exact science.

"Each morning, we wake up and experience a rich explosion of consciousness — the bright morning sunlight, the smell of roast coffee and, for some of us, the warmth of the person lying next to us in bed. As the slumber recedes into the night, we awake to become who we are. The morning haze of dreams and oblivion disperses and lifts as recognition and recall bubble up the content of our memories into our consciousness. For the briefest of moments we are not sure who we are and then suddenly ‘I,’ the one that is awake, awakens. We gather our thoughts so that the ‘I’ who is conscious becomes the ‘me’ — the person with a past. The memories of the previous day return. The plans for the immediate future reformulate. The realization that we have things to get on with remind us that it is a workday. We become a person whom we recognize." - Bruce Hood

In this case, to be conscious is to be self-conscious. It is to know that you are a person who is currently 'active'. In this case, consciousness is lost if you fall asleep. Unless you are experiencing lucid dreams, in which case you know you are who you are and that you are in control of your dreams, but nobody else does. Thus, anyone trying to ascertain whether a sleeping person is conscious or not is facing a problem not unlike Schrödinger's cat. And in this case, as in Schrödinger's, the object of study can get really tired of your experiments and just run off.
Then again, there is no shortage of people reporting out of body experiences. If it were just one or two people among billions, it might be contributed to just normal insanity, but I'd venture to say most of us have had, even remember, at least one episode from our lives where something really bizarre and incomprehensible occurred where we weren't quite... in ourselves. A common neural misfiring no doubt, but a person experiencing such an event, and understanding consciously that he or she is not he nor she at that moment would presumably effectively mean they are unconscious. If that were true, you could be consciously aware that you are unconscious. Whoops.

So how to actually know if a person is conscious? This is made remarkably difficult as the person whose status we are attempting to assess might not just not know whether they are conscious or not, they could consciously be both. I guess what they say is true, "'Normal' is just a cycle in the washing machine.". Perhaps we are all mad. Just going round like a spiral, like a wheel within wheel, never ending or beginning on an ever-spinning reel. Like the circles that you find in the windmills of your mind.