"The zombie scenario goes as follows: imagine that you have a doppelgänger. This person physically resembles you in every respect, and behaves identically to you; he or she holds conversations, eats and sleeps, looks happy or anxious precisely as you do. The sole difference is that the doppelgänger has no consciousness; this – as opposed to a groaning, blood-spattered walking corpse from a movie – is what philosophers mean by a 'zombie'. "Such non-conscious humanoids don’t exist, of course. (Or perhaps it would be better to say that I know I’m not one, anyhow; I could never know for certain that you aren’t.) But the point is that, in principle, it feels as if they could. Evolution might have produced creatures that were atom-for-atom the same as humans, capable of everything humans can do, except with no spark of awareness inside. As Chalmers explained: “I’m talking to you now, and I can see how you’re behaving; I could do a brain scan, and find out exactly what’s going on in your brain – yet it seems it could be consistent with all that evidence that you have no consciousness at all.” If you were approached by me and my doppelgänger, not knowing which was which, not even the most powerful brain scanner in existence could tell us apart. And the fact that one can even imagine this scenario is sufficient to show that consciousness can’t just be made of ordinary physical atoms. So consciousness must, somehow, be something extra – an additional ingredient in nature." -- Oliver Burkeman, "Why can’t the world’s greatest minds solve the mystery of consciousness?"