Qualia
Imagine you're a biochemist born unable to see the color blue—a genetic defect disabling the blue receptors in your retinas. You see only red, green, and their mixtures. Then you discover a cure. Before administering it, you can confidently predict many outcomes: you'll see a new color when you hold up a blue card, you'll call it "blue," you'll experience the same sensation when looking at a clear sky. But there's one thing you cannot predict, that no one can predict: what blue will actually look like.
This is the puzzle of ==qualia—the subjective aspect of sensations.==
You can measure wavelengths with a spectrophotometer, map neural pathways, understand the biochemistry completely. Yet the felt experience itself remains beyond description, beyond prediction. "Qualia are currently neither describable nor predictable," philosopher David Deutsch writes, "a unique property that should make them deeply problematic to anyone with a scientific world view."
The sensation of seeing blue exists in a realm where all our scientific knowledge stops at the threshold of subjective experience.