But what we access may be even more problematic than how we access it: we might, after all, have had immediate access to the physical nature of our conscious states (see . Churchland 1985). But conscious experience instead reveals various sensory qualities—the redness of the visual experience of an apple or the painfulness of a stubbed toe, for example. But these qualities seem to defy informative description. If one has not experienced them, then no amount of description will adequately convey what it’s like to have such an experience with these qualities. We can call this feature of the qualitative component of consciousness indescribability. If someone has never seen red (a congenitally blind person, for example), it seems there is nothing informative we could say to convey to them the true nature of this quality. We might mention prototypical red objects or explain that red is more similar to purple than it is to green, but such descriptions seem to leave the quality itself untouched. And if experienced qualities cannot be informatively described, how could they be adequately captured in an explanatory theory? It seems that by their very nature, conscious qualities defy explanation. This difficulty lies at the heart of the hard problem.
Although it is widely accepted that Descartes explained the problem cogently, few later philosophers have been happy with his solution, and his ideas about the pineal gland have especially been ridiculed.  However, no alternative solution has gained general acceptance. Proposed solutions can be divided broadly into two categories: dualist solutions that maintain Descartes' rigid distinction between the realm of consciousness and the realm of matter but give different answers for how the two realms relate to each other; and monist solutions that maintain that there is really only one realm of being, of which consciousness and matter are both aspects. Each of these categories itself contains numerous variants. The two main types of dualism are substance dualism (which holds that the mind is formed of a distinct type of substance not governed by the laws of physics) and property dualism (which holds that the laws of physics are universally valid but cannot be used to explain the mind). The three main types of monism are physicalism (which holds that the mind consists of matter organized in a particular way), idealism (which holds that only thought or experience truly exists, and matter is merely an illusion), and neutral monism (which holds that both mind and matter are aspects of a distinct essence that is itself identical to neither of them). There are also, however, a large number of idiosyncratic theories that cannot cleanly be assigned to any of these schools of thought.