Hard Problem of Consciousness
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.
Basically, how and why do conscious organisms have experiences, with feelings some of the time, while some don’t.
Qualia
In philosophy, qualia are individual instances of subjective, conscious experience.
From this enlightening TEDRadio Hour podcast
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