From The Three Character Couplet.
“Man at birth
Is quite good
Their nature, similar
but paths diverge.”
“If they do not study
then they cannot know justice.”
Justice is cultivated through virtuous actions which become habitual. It is not natural, even though the fundamental nature of people is good, their character must be cultivated. Neglect of such cultivation is the cause of injustice. But this neglect is in part the fault of the society they live in.