cause and effect
It has to be fair or else the soul becomes restless and peace is disturbed. The greater the perceived injustice the deeper the lack of peace. Thus this is the archetype of karma — cause and effect.
The soul will seek the balance of justice so it will seek to pay for its transgressions and thus learn to live fairly towards others as well as ourselves. And the soul will seek to be paid if it suffered a transgression.
The soul will likewise measure the success of its own achievements and seek to “pay itself” if a goal was not achieved or a plan was not executed.
Justice is a law. It looks squarely at us and is ready to execute it shown by the upright sword. Is the heart as light as a feather? As in the Egyptian story of Maat. If so, we move on to a new cycle in the Wheel of Fortune and can enter the kingdom of heaven. If not, we go back and set things right.