  Perhaps the most basic issue posed by any version of the Why question is whether or not consciousness of the relevant sort has any causal impact at all. If it has no effects and makes no causal difference whatsoever, then it would seem unable to play any significant role in the systems or organisms in which it is present, thus undercutting at the outset most inquiries about its possible value. Nor can the threat of epiphenomenal irrelevance be simply dismissed as an obvious non-option, since at least some forms of consciousness have been seriously alleged in the recent literature to lack causal status. (See the entry on  epiphenomenalism.)  Such worries have been raised especially with regard to qualia and qualitative consciousness (Huxley 1874, Jackson 1982, Chalmers 1996), but challenges have also been leveled against the causal status of other sorts including meta-mental consciousness (Velmans 1991).