Something, (or perhaps nothing) that can't, (or can) be experienced.

Observable things fall into two categories : Things which can be experienced by an unaided human being, and those which can be experienced only by the aid of, or measurement by, an external device.

Thus, the unobservable can not be experienced by a human, or an external device. Some people believe that unobservable things do not exist. Belief in the unobservable usually requires faith.

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