 Knowing about and referring to qualia. Recall that by definition a zombie world is just like our world as physicalists suppose it to be, but without consciousness. Since this implies that consciousness depends on something nonphysical, it follows that zombies (assuming they are possible in the first place) could be made conscious by the addition of something nonphysical, which might as well be qualia. And given that a zombie world would be causally closed, these qualia would have to be causally inert: either epiphenomenal or parallel to the correlated physical processes. It therefore seems that if a zombie world is conceivable then so is epiphenomenalism. (Note that this does not require epiphenomenalism to be actually true as well as conceivable.) If that is correct, objections to the conceivability of epiphenomenalism are also objections to the conceivability of zombies. The most obvious is the familiar and powerful claim that experiences have effects on behavior (Perry 2001). A less obvious objection to epiphenomenalism starts from the fact that we refer to and know about our conscious experiences — which can hardly be denied, since otherwise we could not be discussing these ideas in the first place. The objection appeals to the widely held view that whatever we can know or refer to must have effects on us, even if only indirectly (Kripke 1972/80). On that basis, our counterparts in epiphenomenalistic worlds could not know about or refer to their qualia. If that is right, epiphenomenalistic worlds are not conceivable, in which case neither are zombies.