这句又用了隐喻，把制度比作膳食，护卫者既是制度的制定者，也是恪守者。

But by the same token they will be precluded from cupidity and sordid ambitions; they will always have just so much of the world's goods, and no more; they will be like physicians establishing, and themselves accepting, a dietary for a nation.
但同理，他们无从产生贪婪之心与卑鄙的野心；他们所享有的世间财物，永远是一个定额，不可能更多；他们正如为国民制定膳食方案，并同样恪守这一方案的医生。

细节解析

1、by the same token: for the same reason, in the same way同理
Used to introduce a statement that says something more and often something different about the same situation referred to by a previous statement
By the same token, good bosses will send messages when necessary, not every time a bright idea pops into their head.
2、precluded: to prevent (someone) from doing something阻止；妨碍
When we strive for greatness in our communication, it precludes us from achieving it.
3、cupidity: a strong desire for money or possessions, greed贪婪
I trust indeed that Providence will beat back and humble our cupidity and ambition.《美国历史》
4、sordid: very bad or dishonest卑鄙的，可耻的
It was a sordid business, and I was not inclined to trouble myself with it further. 这件事龌龊不堪，我不想再为它伤脑筋了。《月亮和六便士》
5、dietary: a regulated daily food allowance膳食
Do I understand that he asked for more, after he had eaten then supper allotted by the dietary.我这样理解对吗——他吃完了膳食分配给的晚餐后，又要多来点？《雾都孤儿》

所在段落

The Story of Philosophy《哲学的故事》第1章Plato第7节VI. The Political Solution第6段第3句：

This arrangement will make it unprofitable, as well as dangerous, for the guardians to rule as a clique seeking the good of their class than that of the community as a whole. For they will be protected from want; the necessities and modest luxuries of a noble life will be theirs in regular provision, without the searing and wrinkling care of economic worry. But by the same token they will be precluded from cupidity and sordid ambitions; they will always have just so much of the world's goods, and no more; they will be like physicians establishing, and themselves accepting, a dietary for a nation. They will eat together, like consecrated men; they will sleep together, like soldiers sworn to simplicity. "Friends should have all things in common," as Pythagoras used to say (Laws 807). So the authority of the guardians will be sterilized, and their power made poison less; their sole reward will be honor and the sense of service to the group. And they will be such men as from the beginning have deliberately connected to so materially limited a career; and such men as at the end of their stern training will have learned to value the high repute of the statesman above the crass emoluments of the office-seeking politicians or the "economic man." At their coming the battles of party politics will be no more.
浙江大学译本： 正是出于同样的原因，他们才能够摒弃贪婪和卑鄙的野心；他们永远都只拥有一定量的物质财富，毫不多占；他们像医生一样，一面为国家制订食谱，一面亲身实践着这一食谱。

上句见：Day460