Is the person next to you conscious? It might be impossible to tell, and they could be a zombie – someone who acts exactly like a conscious being, but who has no conscious experience at all.
Philosophers have devised this idea, not necessarilly because they believe zombies exist, but to show that if they did, we currently don’t have the ability to tell them apart from genuinely conscious people.
This is a way of both highlighting the difficulty of defining consciousness, and of having an interesting conceptual tool for exploring the limits of the conscious mind (not everyone agrees, however).
Now, t-shirt company Sebei Industries have created a nifty zombie t-shirt remixed from the Run DMC logo, so you can advertise the fact that you’re actually an unconscious zombie, and save everyone the trouble of having to work it out.
Or maybe you just suffer from walking zombie syndrome?
Link to zombie t-shirt.