孟子以性善說成就其人性論貢獻，這是正面看待一切人類課題的關鍵。然而，就理論本身而言，孟子如何能由人類紛歧矛盾、乃至不善之現象，確立性善？其困難實遠較建構其他人性理論為甚。本文藉《孟子．告子上》第一至六章之文本分析，進行對此之探究。根據析讀結果，六章間具有先後推論關係，涵蓋了人性問題的主要面向。它們分別是：人性論之前提、人善之根據與不善之原因、人性之獨立性、人性與自我孰為真實主體之問題、人性與其現實實踐、對人性本身與人性之道之說明。對照他說，孟子人性論之全面與更高之真實性，更見突顯。

Mencius’ contribution to the debate on human nature is that he advocated that human nature is innately good. This viewpoint is the keystone to a positive appraisal of human existence. But since there are so many conflicts and evils in life, how can Mencius verify this view? Such a standpoint is far more difficult to defend than other theories on human nature. This paper is an analysis of the problem through a rereading of the first six chapters of “Gaozi” in the Mencius. These six chapters are interconnected in their inferences and discuss the important issues regarding this problem. The sequence is as follows: the basis for a theory on human nature, the reasons for the goodness of man and also their evilness, the independency of human nature, the problem of whether human nature or the conscious mind is the true essence of man, human nature and its application in reality, and the explanation of human nature itself as well as the Way of human nature. With such a structure, we can conclude that the view of Mencius on human nature is more complete and profound than all other theories.