Materialism is the view according to which a zombie world is metaphysically impossible. Assuming that zombies are conceivable in the sense that we cannot rule out a priori that our world is a zombie world, materialists must hold that a zombie world is metaphysically impossible despite being conceivable. There are no good reasons to think that this view (type-B materialism) is false, since there are no good reasons to think that the corresponding phenomenal and physical/functional concepts cannot be distinct concepts of the same thing. Nonetheless, we cannot understand how type-B materialism can be true. We cannot understand this, because we cannot in principle explain how a zombie world could be impossible despite being conceivable.