这句解释了上句作法的原因。这些铜铁之人只靠欲望驱动，让他们来守护国家，整个城邦会败坏。正如前面所说，这些只适合参与生产建设，为国家提供经济基础。

For an oracle says that when a man of brass or iron guards the state, it will be destroyed"(415).

解析

1、For an oracle says that when a man of brass or iron guards the state, it will be destroyed"(415).
因为神谕有言：当铜铁之人守护国家时，它必将灭亡。

原文

上句见：Day402
The Story of Philosophy《哲学的故事》第1章Plato第7节VI. The Psychological Solution第11段第13句：

Those who pass this first test will receive ten more years of education and training, in body and mind and character. And then they will face a second test, far severer than the first. Those who fail will become the auxiliaries, or executive aides and military officers of the state. Now it is just in these great eliminations that we shall need every resource of persuasion to get the eliminated to accept their fate with urbanity and peace. For what is to prevent that great unselected majority, in the first test, and that lesser but more vigorous and capable second group of Eliminees, from shouldering arms and smashing this Utopia of ours into a mouldering reminiscence? What is to prevent them from establishing there and then a world in which again mere number or mere force will rule, and the sickly comedy of a sham democracy will reënact itself da capo ad nauseam? Then religion and faith will be our only salvation: we shall tell these young people that the divisions into which they have fallen are God-decreed and irrevocable — not all their tears shall wipe out one word of it. We shall tell them the myth of the metals:
"Citizens, you are bothers, yet God has framed you differently. Some of you have the power of command; and these he has made of gold, wherefore they have the greatest honor; other of silver, to be auxiliaries; others again, who are to be husbandmen and craftsmen, he has made of brass and iron; and the species will generally be preserved in the children. But as you are of the same original family, a golden parent will sometimes have a silver son, or a silver parent a golden son. And God proclaims … that if the son of a golden or a silver parent has an admixture of brass or iron, then nature requires a transposition of ranks; and the eye of the ruler must not be pitiful towards his child because he has to descend in the scale to become a husbandman or an artisan, just as there may be other sprung from the artisan class who are raised to honor, and become guardians and auxiliaries. For an oracle says that when a man of brass or iron guards the state, it will be destroyed"(415).
Perhaps with this "royal fable" we shall secure a fairly general consent to the furtherance of our plan.
浙江大学译本：因为神谕说，一旦铜质或铁质之人掌控了政权，国家的覆亡也就不远了。（第415节）