It's the greatest scientific challenge of our time.
Explaining how something as complex as consciousness can emerge from a grey, jelly-like lump of tissue in the head is arguably the greatest scientific challenge of our time. The brain is an extraordinarily complex organ, consisting of almost 100 billion cells-- known as neurons-- each connected to 10,000 others, yielding some 10 trillion nerve connections.
We have made a great deal of progress in understanding brain activity, and how it contributes to human behavior. But what no one has so far managed to explain is how all of this results in feelings, emotions and experiences. How does the passing around of electrical and chemical signals between neurons result in a feeling of pain or an experience of red?
The problem of consciousness... is radically unlike any other scientific problem. One reason is that consciousness is unobservable. You can't look inside someone's head and see their feelings and experiences. If we were just going off what we can observe from a third-person perspective, we would have no grounds for postulating consciousness at all.
The entire article from livescience.com can be read here.