The Hermit and his baskets.

        “What is the use of my going to hear sermons?” said a hermit to his Superior: “I can never recall what has been said!” the Superior, to show him he always gained some benefit from what he heard, told him to take one of his two baskets and bring him water in it from the river! The hermit was surprised, but obeyed; thrice this was done; the Superior then asked him what difference he found now between that basket and the one he had left untouched. “None,” he said, “unless perhaps the one that has been in the water looks cleaner than the other.”  “just so,” replied the Superior: “as the basket could not retain the water, yet became cleaner each time it was put into the river, so your soul, though perhaps unable to retain all the instruction , yet derives much benefit from every sermon: continue therefore to frequent them.”

Lives of the Fathers.