What is consiousness?

The dictionary defines it as awareness of one's existence. But to what extent, and where lies its root?
I had many arguments with a friend of mine about the subject. He insisted that only humans could be conscious and only after they evolved a language. His reasoning for this was that without a language one can't really think and just engages in automatic behavior. I do not agree with his view. It reminds me of the not-so-long-ago in the past times when many scientists didn't think animals could feel pain and operated on them without anesthetics. I am positive that animals have consciousness. I have a cat, and I observe her thinking, learning, feeling. The matter becomes confusing when you consider primitive creatures and plants.

There is another word: sentient. It is defined as having the ability of feeling or perception. Every living thing appears to be sentient, since every one, no matter how primitive, has an ability to perceive, even without any obvious organs of perception. The perception of plants and molds are mostly chemical or electromagnetic. They definitely react to their environment and are able to adapt. But are they conscious? Are they aware at all of their existence?

When you try to swat a mosquito or a fly, it evades you. It doesn't want to be smashed. Does it feel fear for its life at that moment? I am tempted to say that it does. I realize there are automatic reactions to danger, like the adrenalin rush, in more intelligent creatures, like ourselves, when we bolt or scream before thinking. But whenever that happens we also feel a stab of fear. Do insects feel a stab of fear like ourselves? Mammals do, you can see it in their bodily reactions and expressions. But with bugs we just can't tell. For instance, when you flip a bug on its back, and it waves its legs in the air, is it scared and frantic, or is it just mindlessly waving its legs in order to get a hold of something and to turn right side up? Is it possible, I wonder, to design some sort of an experiment to detect and measure consciousness, to differentiate it from automatic behavior?

I realize, it is hard even for humans to differentiate it in themselves. Many people are mostly unconscious when they go about their daily routine, and in the way they react to various situations. Many people are unconscious when dominated by strong emotions, such as anger. Arguing with them is pointless, they do not hear reason. Are they totally unconscious during those times, or just partially? Is it necessary to be conscious when you perform a complex action such as arguing? We all agree that things like walking are best done unconscious, and we only need to become conscious at times when we evaluate our actions and make a conscious decision to correct them. Do animals evaluate their actions? For instance, when a rat goes through a maze for a second time, is the rat conscious of the wrong turns and does it consciously make a decision not to go that way? Let alone a rat, how about when a slime mold does the maze? A slime mold is only one cell thick and has no sensory organs of any kind, and yet it solves the maze as well as any rat.

I had an interesting experience with a fly once. I was working on my computer, and a fly landed on the edge of the screen. I watched it for a while, then I reached out and touched it - it remained sitting there. It wasn't dazed or sick or anything like that, when I waved at it, it flew off. Did it somehow know that I wasn't going to hurt it and wasn't afraid of me? If that's the case, would that be considered a conscious reaction? I mean, a fly did something different than what it would normally do faced with a human. I mean, wow!