The above sentence appears in "Du's Handbook" as an example sentence in a section that says 爲 (위) can sometimes be translated as "then," similar to how 就 (취) is translated in modern Chinese. However, the character immediately before 爲 is 己 (기), which means "one's own" or 자기 (自己) in Korean, so that makes me wonder "one's own WHAT?" Nothing was mentioned previously, so that suggests a noun should follow 己, not the adverb "then."同於己爲是之, 異於己爲非之“Those [views] which agree with their own they hold to be right, and those which do not so agree they hold to be wrong.”
A couple of days ago, I mentioned that my Korean dictionary says that 爲 (위) can sometimes be translated as "생각하다," which means "to think"; therefore, I think the 己爲 (기위) in this sentence means "자기 생각," which means "one's own (己) thinking (爲)." With that said, here is how I would translate the sentence:
Since the object pronoun 之 (지) follows both 是 (시) and 非 (비), one can assume 是 and 非 should be translated in their transitive verb forms, so I translated 是 as "시인(是認)하다," which means "to approve" or "to acknowledge," and 非 as "비난(非難)하다," which means "to criticize." In Korean the sentence would translate as follows:同於己爲是之, 異於己爲非之“[If it] agrees (同) with (於) their own (己) thinking (爲), [they] acknowledge (是) it (之); [if it] is different (異) from (於) their own (己) thinking (爲), [they] criticize (非) it (之).”
자기들의 생각과 같으면 (同於己爲), 그 것을 시인하고 (是之), 자기들의 생각과 다르면 (異於其爲), 그 것을 비난하는다 (非之).The more I go through "Du's Handbook," the more disappointed I am. The fact that the writers used the above sentence as an example of 爲 (위) being used to mean "then" makes me wonder about other misinformation in the book. I would still recommend the book, but I wouldn't give it "Five Stars." With that said, remember that my opinion is not worth much since I am still just a student trying to teach myself Classical Chinese.