The easy problems, and the one hard one
In 1995, the philosopher David Chalmers drew a line that has shaped the field ever since. On one side sit what he called the easy problems of consciousness: explaining how the brain discriminates a stimulus, integrates information, focuses attention, controls behavior, and produces verbal reports. These are not trivial, but they are the kind of thing science already knows how to attack. Each one asks how a function is performed, and in principle a mechanism can answer it.
On the other side sits the hard problem. Even after every function is explained, one question stubbornly remains: why is all that processing accompanied by inner experience? Why is there something it is like to see red, taste coffee, or feel pain, rather than the information being handled 'in the dark' with no felt quality at all? A being that behaved exactly like you but had no inner life, a so-called philosophical zombie, seems at least conceivable, and that is precisely what makes the problem hard.
What is it like to be a bat?
The intuition behind the hard problem was sharpened two decades earlier by Thomas Nagel. In his 1974 essay 'What Is It Like to Be a Bat?', Nagel argued that an organism is conscious if there is something it is like to be that organism, from its own point of view. A bat navigates by echolocation, a sense we do not share. We can learn everything about bat physiology, yet we still cannot know what the bat's world feels like from the inside.
Nagel's point was general. Every conscious experience has a subjective character that is tied to a particular point of view, and science trades in the objective, the view from nowhere. That mismatch, he suggested, is why physical accounts of the mind can feel like they leave something out, no matter how complete they become.
The explanatory gap, and those who doubt it
Joseph Levine gave this missing link a name in 1983: the explanatory gap. Even if pain simply is a certain pattern of neural firing, that identity does not seem to explain why the firing hurts. Levine himself was cautious, treating the gap as a limit on our understanding rather than proof that experience is non-physical.
Not everyone agrees the problem is real. Illusionists such as Keith Frankish and Daniel Dennett argue that phenomenal consciousness, in the rich sense the hard problem assumes, is a kind of user-illusion the brain generates about itself; the real task is to explain why we are so convinced we have it. Chalmers has reframed exactly that challenge as the 'meta-problem': explain our reports and intuitions about consciousness. Realists reply that the felt quality is the one thing we cannot be wrong about. The disagreement is live, and honest observers admit no experiment has yet settled it.