Rabbi Perida had a pupil to whom he had to rehearse a lesson four
hundred times before the latter comprehended it. One day the Rabbi was
hurriedly called away to perform some charitable act, but before he went
he repeated the lesson in hand the usual four hundred times, but this
time his pupil failed to learn it. "What is the reason, my son," said he
to his dull pupil, "that this time my repetitions have been thrown
away?" "Because, master," naively replied the youth, "my mind was so
pre-occupied with the summons you received to discharge another duty."
"Well, then," said the Rabbi to his pupil, "let us begin again." And he
repeated the lesson a second four hundred times.

THE TALMUD, _Eiruvin_, fol. 54, col. 2.