A solved problem.
A defined problem believed to have a solution.
A defined problem whose solution may be impossible.
A defined problem believed to have no possible solution.
An undefined but acknowledged felt difficulty believed to contain a definable problem.
An undefined but acknowledged felt difficulty which might contain a definable problem.
An undefined but acknowledged felt difficulty believed to be essential to existence.
An unacknowledged felt difficulty.
An unfelt difficulty: a non-problem.
A solved problem?

Light: unobstructed perception.
Shadow: darkness in light, obstructed perceiving.
Darkness: obstructed perception, nonperception in perception.
Blindness in darkness: obvious obscurity, perceived nonperception.
Blindness in light: nonobvious obscurity, blindspots, unperceived nonperception.
Light: unobstructed perception?