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The Salmon of Knowledge (Irish: bradán feasa) is a creature figuring in the Fenian Cycle of Irish mythology. (It is sometimes identified with Fintan mac Bóchra, who was known as "The Wise" and was once transformed into a salmon.)
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The Salmon story figures prominently in The Boyhood Deeds of Fionn, which recounts the early adventures of Fionn mac Cumhaill. According to the story, an ordinary salmon ate nine hazelnuts that fell into the Well of Wisdom (an Tobar Segais) from nine hazel trees that surrounded the well. By this act, the salmon gained all the world's knowledge. The first person to eat of its flesh would in turn gain this knowledge.
The poet Finn Eces (or Finegas) spent seven years fishing for this salmon. Finally Finn caught the salmon and gave the fish to Fionn, his servant and son of Cumhaill, with instructions to cook it but on no account to eat any of it. Fionn cooked the salmon, turning it over and over, but when he touched the fish with his thumb to see if it was cooked, he burnt his finger on a drop of hot cooking fish fat. Fionn sucked on his burned finger to ease the pain. Little did Fionn know that all of the salmon's wisdom had been concentrated into that one drop of fish fat. When he brought the cooked meal to Finn Eces, his master saw that the boy's eyes shone with a previously unseen wisdom. Finn Eces asked Fionn if he had eaten any of the salmon. Answering no, the boy explained what had happened. Finn Eces realized that Fionn had received the wisdom of the salmon, so gave him the rest of the fish to eat. Fionn ate the salmon and in so doing gained all the knowledge of the world. Throughout the rest of his life, Fionn could draw upon this knowledge merely by biting his thumb. The deep knowledge and wisdom gained from the Salmon of Knowledge, allowed Fionn to become the leader of the Fianna, the famed heroes of Irish myth.