In ancient Greek culture, Dike ("justice") was the spirit of moral order and fair judgement based on immemorial custom, in the sense of socially enforced norms and conventional rules. According to Hesiod, she was fathered by Zeus upon his second consort, Themis.
She ruled over human justice, while her mother Themis ruled over divine justice. Her opposite was adikia ("injustice").
The later art of rhetoric treated the personification of abstract concepts as an artistic device, which devolved into the allegorizing that Late Antiquity bequeathed to patristic literature. In a further euhemerist interpretation, Dike was born a mortal and Zeus placed her on Earth to keep mankind just. He quickly learned this was impossible and placed her next to him on Mount Olympus.
Lady Justice is most often depicted with a set of scales typically suspended from her right hand, upon which she measures the strengths of a case's support and opposition. She is also often seen carrying a double-edged sword in her left hand, symbolizing the power of Reason and Justice, which may be wielded either for or against any party.