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). 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 normal sight, and 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 Gaidhlig speakers, even though, strictly speaking, it does not really mean second sight. It originated in the Scottish Highlands.
History of symbolic visions
Perhaps our earliest notice 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.
Ranulf Higdons Polychronicon (14th century) describes Scottish second sight, adding "that strangers setten their feet upon the feet of the men of that londe for to see such syghtes as the men of that londe doon".