He said:

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I have written elsewhere that all good deeds are justice and all evil deeds are injustice, and that Allah sent down the Scriptures and sent the messengers only so that people might uphold justice.

I have also explained that justice and injustice fall into two categories: one concerns the rights of Allah Most High—as in affirming Allah’s oneness, which is the foundation of justice, whereas associating partners with Him is the foundation of injustice. The other concerns people’s rights, either together with Allah’s rights, as in taking a life, or on their own, as in a debt incurred with the creditor’s consent.

Injustice concerning people’s rights is itself of two kinds. One occurs without the consent of the person concerned, such as taking his life, seizing his property, or violating his honour. The other occurs with his consent but is nonetheless unjust, as in transactions involving usury or gambling. These are forbidden because they involve consuming another person’s wealth wrongfully, and consuming wealth wrongfully is injustice.

Even if its owner consents, this does not become permissible or cease to be unjust. Thus, not everything to which its owner freely consents ceases to be unjust, nor is everything a person parts with unwillingly an injustice. Rather, there are four categories: