A molecule for molecule replica/duplicate of a human being. The difference is that it lacks conscious experience i.e. qualia.
If philosophical zombies are identical and functionally isomorphic to human beings, then this causes a problem with regards to knowledge of other minds i.e. we cannot be sure that others have minds, since they might be a zombie who lacks qualia.
It doesn’t matter whether they exist or not, but since they are conceivable, it is a logical possibility for philosophical zombies to exist. If zombies are possible, then physical facts alone are not enough to explain consciousness. Therefore disprove materialist reductionist theories such as Behaviourism, Identity Theory and Functionalism - they must be false.
It can be used in support of Substance Dualism
say that mental states are analytically reducible to actual or potential behaviours, but if a philosophical zombie acts and behaves in exactly the same way as a human being, there is no way of accounting for the conscious experience that we intuitively feel we have.
Against Identity theory
we can conceive of a zombie that has a brain like a human, and yet even though they have the same brain states (e.g. c-fibres firing) they are missing that inner qualitative experience of what it is like to be in a certain mental state.
we can imagine a functionally isomorphic zombie with inputs, internal processes, and outputs exactly the same as human beings, but does not have consciousness.
she learns everything there is to possibly know about what the colour red is, the physical processes that occur within our retina, brain etc. understands the principles of light refraction etc., but she has never actually experienced ‘red’ i.e. she has never had that qualitative experience of being in the state of sensing ‘red’. She is learning something new when she sees it. There is something over and above our knowledge.
just because we can conceive of a zombie, doesn’t necessarily mean that zombies can exist. An epistemological claim is being confused with an ontological claim. You can’t draw an ontological conclusion, from an epistemological claim.
e.g. We can conceive of a triangle without having the property of the Pythagorean theory, but that does not necessarily mean that the triangle lacks it, or that it is not an essential property of that triangle.