Imagine a being that acted exactly like a human, it spoke, laughed, etc. - everything that you'd imagine a human being to do. However, it does not actually feel or think.
For example, if you were to prick one of these "zombies," they would say "ouch," recoil, perhaps berate you, but the zombie does not actually feel pain - it's a purely mechanical process.
Metaphysics is concerned with the nature of being, properties, identity, etc. What one means by whether such zombies are metaphysically possible is really an answer to the question: are such zombies really any different than how we are?
For those of you interested in learning more, look up these terms (I studied philosophy related to Complex Systems, and zombies were a central question in my thesis):
John Searle's Chinese Room, Turing Test, physicalism, Conway's Game of Life
It is. A lot of what metaphysics is concerned with now has a lot to do with AI.
You can think of zombies as the analog to robots (as is canonical with most AI questions). Robots are essentially philosophical zombies, should we get them to a level of sophistication such that they would be indistinguishable from a human being. Whether such robots could be considered "living," "thinking," or "feeling," is a question to be answered by philosophy.
If you're more interested read about John Searle's Chinese Room thought experiment. It gives a great example of what you're thinking of.
Imagine a being that acted exactly like a human, it spoke, laughed, etc. - everything that you'd imagine a human being to do. However, it does not actually feel or think.
For example, if you were to prick one of these "zombies," they would say "ouch," recoil, perhaps berate you, but the zombie does not actually feel pain - it's a purely mechanical process.
Metaphysics is concerned with the nature of being, properties, identity, etc. What one means by whether such zombies are metaphysically possible is really an answer to the question: are such zombies really any different than how we are?
For more: http://en.wikipedia.org/wiki/Philosophical_zombie