The hard problem of consciousness is the problem of explaining how and why sentient organisms have qualia[note 1] 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. The philosopher David Chalmers, who introduced the term “hard problem” of consciousness, contrasts this with the “easy problems” of explaining the ability to discriminate, integrate information, report mental states, focus attention, and so forth. Easy problems are (relatively) easy because all that is required for their solution is to specify a mechanism that can perform the function. That is, regardless of how complex or poorly understood the phenomena of the easy problems may be, they can eventually be understood by relying entirely on standard scientific methodologies. Chalmers claims that the problem of experience is distinct from this set and will “persist even when the performance of all the relevant functions is explained”.
The existence of a “hard problem” is controversial. It has been accepted by philosophers of mind such as Joseph Levine, Colin McGinn, and Ned Block and cognitive neuroscientists such as Francisco Varela, Giulio Tononi, and Christof Koch, but disputed by philosophers of mind such as Daniel Dennett, Massimo Pigliucci, and Keith Frankish and cognitive neuroscientists such as Stanislas Dehaene, Bernard Baars, and Antonio Damasio.
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Prof. Donald Hoffman gives an interesting perspective on the ‘hard problem’ in his work on Conscious Agent Theory: see also:/prof-donald-hoffman-on-conscious-agent-theory