Zombies are dead behind the eyes, and stagger around seeking sweet, sweet brains. In the context of philosophy, the staggering around is less important - what philosophers focus on is the notion of a being lacking in conscious experience.
A ‘philosophical zombie’ is a being that physically exactly resembles a normal, healthy human being, but inwardly does not experience the conscious experiences that human beings ordinarily experience. Zombies can think, talk and act, but they do not experience sensations of colour, taste, etc. When we see a blue sky, or a red apple, we get a direct impression of blue-ness or red-ness - there is ‘something it is like’ to see a blue sky. This ‘what it is like’ element is a vital part of conscious experience. Yet the zombie, though it might claim that it has rich experiences of blue, and rich tastes of sweet and sour, does not experience these experiences. It is brain-dead in the sense that there is nothing going on inside its head in terms of conscious experience. The main thing to note is that the zombie is identical to any other human being in terms of physical constitution - but lacks rich conscious experiences of things such as colour, sound and taste.
|Daniel Dennett claims that consciousness|
is largely an illusion. Perhaps this is
because he is a zombie. (Chalmers)
the physical world than simply physical stuff - that consciousness indicates that there is also mental stuff. (See previous post: the Explanatory Gap). Though zombies probably do not exist in the actual world, the idea of a zombie seems possible - it seems like a possibility that zombies could exist in the real world, though if they did we would not know that they were zombies.
If it is possible that zombies exist, then physicalism/materialism, the view that everything that exists is composed of physical matter, is false. If zombies are possible, then a being physically identical to a healthy conscious human being, but which lacks conscious experience, is possible. If such a being is possible, then consciousness is not fully determined by the physical. And if consciousness is not fully determined by the physical, then whatever accounts for consciousness is non-physical. Hence, physicalism is false.
Whether we give any attention to such a thought experiment depends on our individual intuitions - in my opinion, it seems quite plausible that a physical copy of myself could lack the conscious experiences that I have. But you might not agree.