The following video is of an interview with neuroscientist Giulio Tononi in which Tononi explains what's called the "hard problem" of human consciousness.
Consciousness, our awareness of our experience of the world, has proven to be extremely difficult to explain. How do electrochemical goings-on in the brain produce the sensation of blue, or the taste of sweet, or the sound of a violin?
No one knows, and our inability to explain consciousness in material terms has led many philosophers to doubt that materialism provides a correct or complete picture of human beings.
Many have argued that in addition to our physical material brains there must be something else, an immaterial substance, mind or soul, that somehow plays a role in producing sensations like pain, color, warmth, and so on.
Indeed, the nature of consciousness is perhaps the deepest mystery in biology, and this video helps give some insight why that's so: