这句通过“But…, let it be …; one would rather...”形成了完整的链条：提出误解——郑重纠正——给出更准确的新定义。这个结构值得学习套用，
西方中贵族往往是无所事事，且继承爵位的那些人，西方的历史告诉我们没有什么人喜欢，除了其利益既得者。

But we have come to think of aristocracies as hereditary; let it be carefully noted that this Platonic aristocracy is not of that kind; one would rather call it a democratic aristocracy.
然而，我们已然将贵族制视为种世袭的制度。需谨记，这种柏拉图式的贵族制并非如此，或可称之为民主式贵族制。

细节解析

1、come to think of: 它包含“逐渐形成共识”的过程感，译为“通常认为”或“已然惯于将……视为……”更精准。
Really, when one comes to think of it, an astonishing fact, a fact vastly significant.真的，当人们想到它时，认为是个惊人的事实，一个极其重要的事实。《四季随笔》

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2段第4句：

Is this aristocracy? Well, we need not be afraid of the word, if the reality is good which it betoken: words are wise men's counters, without value of their own; they are the money only of fools and politicians. We want to be ruled by the best, which is what aristocracy means; have we not, Carlyle-like, yearned and prayed to be ruled by the best? But we have come to think of aristocracies as hereditary; let it be carefully noted that this Platonic aristocracy is not of that kind; one would rather call it a democratic aristocracy. For the people , instead of blindly electing the lesser of two evils presented to them as candidates by nominating cliques, will here be themselves, every one them, the candidates; and will receive an equal chance of educational election to public office. There is no caste here; no inheritance of position or privilege; no stoppage of talent impecuniously born; the son of a ruler begins on the same level, and receives the same treatment and opportunity, as the son of a bootblack; if the ruler's son is a dolt he falls at the first shearing; if the bootblack's son is a man of ability the way is clear for him to become a guardian of the state(423). Career will be open to talent wherever it is born. This is a democracy of the schools — a hundredfold more honest and more effective than a democracy of the polls.
浙江大学译本： 但是，贵族体制是一种世袭的制度，在这里必须清楚地指出，柏拉图式的贵族体制并非如此；或许它更应被称作是民主式贵族政体。

上句见：Day437