Thus in the Conferences of the Fathers xxiv, 21, it is related of Blessed John the Evangelist, that when some people were scandalized on finding him playing together with his disciples, he is said to have told one of them who carried a bow to shoot an arrow. And when the latter had done this several times, he asked him whether he could do it indefinitely, and the man answered that if he continued doing it, the bow would break. Whence the Blessed John drew the inference that in like manner man's mind would break if its tension were never relaxed.
Thomas Aquinas, Summa Theologiae 2-2.68.2
