We often think of justice in legal terms, as in the justice system or someone being a justice of the peace. But justice is not inherently a matter of laws and rules. Rather, our laws exist to serve justice (they should, anyway), and the justice system should serve to enforce just laws. Getting to the heart of the matter, then, what is justice? Turning to the Catechism once again, we find the following definition: 
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Justice is the moral virtue that consists in the constant and firm will to give their due to God and neighbor. Justice toward God is called the “virtue of religion.” Justice toward men disposes one to respect the rights of each and to establish in human relationships the harmony that promotes equity with regard to persons and to the common good. The just man, often mentioned in the Sacred Scriptures, is distinguished by habitual right thinking and the uprightness of his conduct toward his neighbor. “You shall not be partial to the poor or defer to the great, but in righteousness shall you judge your neighbor.” “Masters, treat your slaves justly and fairly, knowing that you also have a Master in heaven.”

In a word, justice is giving God and neighbor what they are owed. While fortitude and temperance are focused on self-control, justice is critical because it governs how we relate to both God and our neighbor. 
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In scripture, we are told to “do justly” and walk humbly with God (Micah 6:8). Justice isn’t just negative, a matter of dealing out judgment, but also a positive thing, in that we render whatever is owed to God and others.  

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