From Garfunkel's memorable solo LP, Angel Clare. His voice was at the height of its powers when this was recorded in 1973. The tune was written by Jimmy Webb.
The late and wonderful Larry Knechtel, a studio musician who also performed with a group of fellow studio musicians in the band, Bread, played the piano. Knechtel's most famous piano solo came in the Simon and Garfunkel tune recorded about four years before All I Know. That song was Bridge Over Troubled Water.