The hard problem of consciousness is the problem of explaining how and why sentient organisms have qualia or phenomenal experiences—how and why it is that some internal states are felt states, such as heat or pain, rather than unfelt states, as in a thermostat or a toaster.^[1] The philosopher David Chalmers, who introduced the term “hard problem” of consciousness,^[2] contrasts this with the “easy problems” of explaining the ability to discriminate, integrate information, report mental states, focus attention, etc. Easy problems are easy because all that is required for their solution is to specify a mechanism that can perform the function. That is, their proposed solutions, regardless of how complex or poorly understood they may be, can be entirely consistent with the modern materialistic conception of natural phenomena. Chalmers claims that the problem of experience is distinct from this set and that the problem of experience will “persist even when the performance of all the relevant functions is explained”.^[3]

The existence of a “hard problem” is controversial and has been disputed by philosophers such as Daniel Dennett^[4] and cognitive neuroscientists such as Stanislas Dehaene.^[5]

More at:  https://en.wikipedia.org/wiki/Hard_problem_of_consciousness