本論文以異作為分離相論的轉折這點為主軸，逐段解釋一些相關的段落來看異之相在晚期相論中的作用，主要涉及的對話錄是《哲人篇》與《巴門尼德斯篇》。 本論文的架構是從《巴門尼德斯篇》第一部分開始，看分離相論所會導致的問題，再從第二部分的回應中看到分離以外，假定諸相相互結合作為另外一種可能，以及在第五組假設中所顯現的異之相所具有的存有學上的重要性。而通種論中最高種間彼此的關係、在《哲人篇》中所引入的二分法以及篇中所連帶解決的問題則在二到五章中呈現。並於第五章節總結了異在《哲人篇》及通種論中的地位及作用。首先是異在《哲人篇》中達到了指出有一種虛假的言說存在的目的；並且異的存在解釋了諸相的基本結構，最高種的劃分必也是依據這種基礎，它們必分有「是」而是，分有異而得以相區別，而諸種屬必相連結構成那諸多被區別出的種或類；回返至《哲人篇》前半部和最後一部分所使用的分解法，此方法亦是立基於通種論和異的結構上。通過一個整體的連結能夠有一個包含其下所有類的最高種，其得以作為一個劃分的起點，而類差作為劃分的依據也因同異的存在而是可能的。 / The aim of this thesis is to show how the idea of difference works in Plato’s late theory of Ideas, and it is chiefly concerned with Plato’s Parmenides and Sophist. The theme of the first part of Parmenides is what will be caused by Chorismos, and of the second part might show another possibility from the Chorismos, that is, the communion of genera. And then the fifth hypotheses of the second part of Parmenides might show the importance of the idea of difference in Plato’s late ontology. In Sophist, since the theory of communion of genera might solve the problem of Chorismos, and the idea of difference as one of the greatest genera of the communion of genera, which might be the most important one, I deal with it and the relations between the fives greatest genera. There are also the problems that be solved by the idea of difference in Sophist, and which brings a new method (dialectics) of Plato’s late dialogues, for defining all of them clearly, I analysis those concerned paragraphs.