But think not that thus this taking away, could be by means of cutting off or separation; but it is here, as is the case in an operation effected by fire, which can light ten thousand torches, without itself being diminished the least atom, or ceasing to remain as it was before. Something like this also is the nature of knowledge. For though it has made all its pupils, and all who have become acquainted with it, learned, still it is in no degree diminished itself, but very often it even becomes improved, just as, they say, that fountains sometimes are by being drained dry; for, it is said, that they sometimes become sweeter by such a process.