In mythology, the Well of All Knowledge stood on a hill owned by the god Nechtan. The Salmon of Knowledge swam in it, eating the Nuts of Knowledge that fell from nine sacred Hazel trees that surrounded the Well. Nechtan and his three cupbearers were the only people who could visit it.
When Nechtan’s wife Boann attempted to drink from the Well, it burst and formed the River Boyne which was subsequently named after the goddess. When this happened, the Salmon of Knowledge was washed from the Well into the River Boyne. According to ancient prophecy, the first person who tasted the Salmon’s flesh would enjoy the wisdom of the ages.
The Druid Finnegas spent seven years trying to catch the Salmon and during that time taught his apprentice Fionn mac Cumhaill how to prepare and cook for him. Finnegas believed that one day the Salmon would be his and told Fionn nothing of its significance.
When Finnegas finally caught the Salmon he took it to Fionn to cook, warning him not to touch its skin. But when Fionn was cooking the fish a blister rose on its side and, without thinking, Fionn touched the Salmon to burst the blister. A piece of the Salmon's skin stuck to Fionn’s thumb and when he stuck his thumb in his mouth to ease the pain the Salmon’s knowledge was transferred to him. Fionn mac Cumhaill went on to become a legendary warrior and healer. Anytime he wished to divine the future or see distant events, he had only to put his thumb into his mouth and the desired knowledge would be his.
A similar story is found in Welsh and Scottish legend.