who said the greatest thing you'll ever learn
It is the last line of a song written by Eden Ahbez.
Nature Boy
 But he believed that it was the song's last line that made it the most poignant: "The greatest thing you'll ever learn, Is just to love and be loved in return".
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Nature Boy [SEP] Recording and composition
Author Raymond Knapp described the track as a "mystically charged vagabond song" whose lyrics evoked an intense sense of loss and haplessness, with the final line delivering a universal truth, described by Knapp as "indestructible" and "salvaged somehow from the perilous journey of life". According to Joe Romersa, an engineer/drummer in Los Angeles, to whom ahbez bequeathed master tapes, photos, and final works, ahbez wanted a correction made to the lyrics saying "To be loved in return, is too much of a deal, and there's no deal in love.", and that instead it should read "The greatest thing you'll ever learn is to love and be loved, just to love, and be loved." Romersa has stated that, because these lyrics did not fit with the original ending melody, ahbez re-wrote it.