As a seven-year-old boy, Finn Mac Cuhaill met on the banks of the Boyne with a seer called Finneigeas. Finneigeas had dedicated the past seven years of his life to catching the Salmon of Knowledge, which swam in the river and would impart the knowledge of the world on the first person to taste it.
While Finn was there, Finneigeas caught the salmon, and with much joy put it on the spit to cook, entrusting the cooking to Finn but warning him not to taste it. After a time, Finn went to see if the fish was cooked, however, he touched it with his thumb and burnt himself, leaving a blister. To ease the pain, he put his thumb in his mouth, and thus became the first person to taste the salmon. When Finneigeas looked at the boy’s face, he saw the wisdom shining in it, and knew that the salmon was no good to him. Ever after that, if Finn needed to know something, he put his thumb into his mouth and the knowledge came to him.