孟子所處的戰國時期，有許多對於人性的理解。孟子認為天下人談論事物的本性，都是就既成的事實來說，既成的事實以順從自然為基礎。依據《孟子》的記載，當時與孟子不同的人性論至少有三種，皆反映了人類生活的既成事實：主張「性無善無不善」、「性可以為善，可以為不善」兩種觀點的思想家，認為人的行為與人性受到環境決定；持「有性善，有性不善」觀點者，認為人性善惡與生俱生。孟子的性善論則洞見人心的作用，可以認可或否定環境與欲望作為自己行動的理由，因此人能夠決定自己的行為，也為人帶來責任。行善則心安理得，不行善則不安，來自內心的壓力就是最直接的懲罰。最後，本文指出即使行為受環境決定別無選擇，行為者仍然可能為行為負責。

Being in the time of the Warring States period, Mencius had much realization among human nature. He considers that when people talk about objects, the nature is to make discussions based on existing facts. The existing facts follow the nature as its foundation. According to Mencius, during Warring States period, there were at least three different theories of human nature besides Mencius’. All of them reflect the existing facts in human life. Thinkers who advocate “human nature is neither good nor bad” and “human nature may be made to practice good, and it may be made to practice evil” consider that the behavior and nature of human are determined by the environment. Those who think “some have good human nature while some don’t” assume that people are born with the good or evil human nature. Mencius’ theory of good human nature gives an insight of human hearts. It is up to human to recognize or deny the environment and desire as the reason for one’s actions. Thus human are able to determine their own behaviors and they bear the responsibility that comes with it. If one does good, they will have peace of mind and vice versa. The pressure from their heart is the most straightforward and direct punishment. Lastly, it is stated in this article that even one’s behavior is affected by the environment as there seems to be no other choice left, the one taking action might still be responsible for their actions.