在柏拉圖《費都篇》(Phaedo)裡，蘇格拉底對他以前的自然哲學家們有所不滿，因為他們只用自然裡的基本粒子或最原始的物質做為原因，來解釋萬有的生滅，因而他假設相做為原因，來從事靈魂不朽的研究。根據亞理斯多德的記載，蘇格拉底只歸納那些言談和定義普遍者，他並未使普遍者分離，也未曾主張分離的萬有或者相。由這兩個對蘇格拉底的記載之間的差異，筆者試圖重新釐定從蘇格拉底到柏拉圖之間的哲學發展。本文用以下的幾個步驟來進行這個論題的研究：一、方法學上的考察：自然對象與倫理對象之間的差異及其與理性、感覺之間的關係，二、相的假設和倫理對象之間的關係，二、相的假設和倫理對象之間的關係以及相的獲得，三、結論。 在第一個步驟裡，筆者先分析「倫理對象」的特點，然後依據感覺及理性之間的不同，討論「自然對象」、「感覺對象」、「倫理對象」及「知識對象」之間的異同。 在第二個步驟裡，筆者先討論以下的四點：一、自在的倫理相的假定及其特徵。二、自在的倫理相對各個的倫理行為的蘊含關係。三、個別的倫理行為對自在的倫理相的分有關係。四、自在的諸倫理相之間的相互關係。然後依據陳康先生的研究，筆者整理出柏拉圖對於相的獲得的理論。 從本文的研究，可以整理出以下的幾點結論：一、蘇格拉底將倫理對象視為理論對象，但是這倫理對象不是相。他認定倫理對象能夠獨立於感覺對象之外而獲得普遍定義。二、柏拉圖通過相的假定，使得知識成為永恆不變的研究對象。三、用相做為與之同名的個別事物之原因，柏拉圖稱此原因為：「分有」。同名或自我指涉以及相與個別事物如何分有－這成為相論的幾個理論困境。四、根據陳康先生的講法，柏拉圖認為：藉著這些不同的認知方式而可以獲得相的認知。五、柏拉圖晚年的相論奠基於分解法，通過分解法的運用，可釐定相與相或相與種之間存在的蘊含關係。

　　　　　According to Plato's Phaedo, Socrates doesn't content himself with the presocratic physiologists who explain the causes of being with material element or voυ? like anaxagoras, so that he assumes the existence of ideas as the cause of γενεσι? in order to investigate the immortality of soul. In comparison with Aristotle's report, Socrates reduces his discussions with sophists into definitive topics and searches for the καθολον in his definition of the ethical, but he neither asserts the separation of καθολον nor affirms the existence of ideas. Owing to this difference of the delivered reports between Plato and Aristotle, I try to investigate the development from Socrates' διαλεγεσθαι to Plato's theory of idea. This paper is divided into three steps as following: 1.A methodical consideration: on the differences between the natural and the ethical in reason and sensation. 2.On the hypothesis of ideas, its relation to the ethical and the acquisition of ideas. 3.conclusion. By the first step, I analyze the characteristics of the ethical and put them into the division of reason and sensation in order to make distinctive marks between them. By the second step, I inquire the hypothesis of ideas into the following points of view: 1.on the hypothesis of ideas themselves and their characteristics, 2.on the implication of ideas, 3.on the μεθεσι? of ideas and the participants, 4.on the relation between ethical ideas. Afterwards, I summarize L. C.-H. Chen's study of 'Ideenschau' in his last work: Acquiring Knowledge of the Ideas. Consequently, it concludes as following: 1.Socrates considers the ethical as theoretical and sets down the definitive boundary of it without separating it form the sensible. 2.Out of Socrates' καθολον, Plato hypothesizes the existence of ideas and makes the objects of knowledge insensible and perpetual. 3.Theμεθεσι? is this explanation of causality. 4.According to L. C.-H. Chen, it admits of the acquisition of ideas in Plato's theory of ideas. 5. In Plato's later theory of ideas, he uses the method of διαρετικη by which the relation between ιδεαι and γενοιand γενοι can be distinctively marked out.