Second sight is a form of extrasensory perception, the supposed power to perceive things that are not present to the senses, whereby a person perceives information, in the form of a vision, about future events before they happen (precognition), or about things or events at remote locations (remote viewing).
There is no scientific evidence that second sight exists. Reports of second sight are known only from anecdotal evidence given after the fact.
Second sight may have originally been so called because normal vision was regarded as coming first, while supernormal vision is a secondary thing, confined to certain individuals.An da shealladh or "the two sights," meaning "the sight of the seer", is the way Gaels refer to "second sight", the involuntary ability of seeing the future or distant events. There are many Gaelic words for the various aspects of second sight, but an da shealladh is the one mostly recognized by non-Gaelic speakers, even though, strictly speaking, it does not really mean second sight, but rather "two sights".
An early example of symbolical second sight is found in the Odyssey, where Theoclymenus sees a shroud of mist about the bodies of the doomed Suitors, and drops of blood distilling from the walls of the hall of Odysseus. The Pythia at Delphi saw the blood on the walls during the Persian War; and, in the Argonautica of Apollonius Rhodius, blood and fire appear to Circe in her chamber on the night before the arrival of the fratricidal Jason and Medea. Similar examples of symbolical visions occur in the Icelandic sagas, especially in Njala, before the burning of Njal and his family. In the Highlands, and in Wales, the chief symbols beheld are the shroud, and the corpse candle or other spectral illumination.