 Explanans. The range of possible explanans is also diverse. In perhaps its broadest form, the How question asks how consciousness of the relevant sort could be caused or realized by nonconscious items, but we can generate a wealth of more specific questions by further restricting the range of the relevant explanans. One might seek to explain how a given feature of consciousness is caused or realized by underlying neural processes, biological structures, physical mechanisms, functional or teleofunctional relations, computational organization, or even by nonconscious mental states. The prospects for explanatory success will vary accordingly. In general the more limited and elementary the range of the explanans, the more difficult the problem of explaining how could it suffice to produce consciousness (Van Gulick 1995).