If one wishes to look deeply into the nature of the human mind and to search out its inner sources of delight, he will find there something of strength conjoined with something of weakness, and out of this circumstance arises variety and irregularity. The mind's vexation with a continual relaxation derives from its strength, while from its weakness stems the fact that it cannot bear a continual straining. Hence it is that nothing pleases the human mind very long, nothing that is all of one piece. So in music it rejects a wholly perfect harmony, and for this reason musicians deliberately intercalate discordant sounds--what are technically called dissonances. So, finally, it happens that physical exercise, even if it was at first undertaken for pleasure, becomes a torture when continued without interruption. This point has its pertinence to literature, the more so since in that field nature reveals the greatest delicacy and cannot long endure what is lofty and excited. Yet on the other hand, whatever creeps close to earth and never lifts its head is, if it be prolonged, wearisome. To stand, to rest, to rise up, to be thrown down, this is what every reader or listener desires, and from this derives the driving necessity for variety, for the mingling of the majestic and slight, excited and calm, high and low. But it may seem that this consideration has little pertinence to the epigram, which is brief and so in less need of variety. However, I need not apologize for introducing these more general considerations since others of more immediate pertinence to the course of our discussion are derived from them, and particularly the question of the discriminate use of metaphors, which are of considerable effect in adorning or vitiating poetry.