‘Adl (fairness and justice) is i’tidaal (balance), and in balance lies the correction of the heart, just as in dhulm (imbalance/oppression) lies its corruption. This is why for every sin that the person has committed he has oppressed his self (dhaaliman li nafsihi). The opposite of dhulm is ‘adl, so this sinful person has not been just to his self, rather he has oppressed it. The correction of the heart lies in ‘adl and its corruption lies in dhulm. Therefore, when the servant oppresses himself he is the oppressor and the oppressed at the same time, likewise when he is just then he is the one who is just and the one upon whom the justice is carried out.
‘Aml (actions) have an effect upon the heart, either of benefit, harm, or correction, before they effect the external body. The good and pure actions constitute justice for the soul whereas bad actions oppress the soul. Allaah the Most High, said,
مَنْ عَمِلَ صَالِحًا فَلِنَفْسِهِ وَمَنْ أَسَاء فَعَلَيْهَا
Whosoever does righteous deeds it is for the (benefit of) himself, and whosoever does evil, it is against his ownself. (Surah Fussilat 41:46)
إِنْ أَحْسَنتُمْ أَحْسَنتُمْ لِأَنفُسِكُمْ وَإِنْ أَسَأْتُمْ فَلَهَا
If you do good, you do good for your ownselves, and if you do evil, (you do it) against yourselves. (Surah al-Israa’ 17:7)
Some of the Salaf said, ‘Indeed good actions are a light in the heart, a strengthening for the body, a glow on the face, a cause for extensive provisions and love in the hearts of the creation. Indeed bad actions are a darkness in the heart, a blackness on the face, a weakness for the body, a cause for decrease in provisions and hatred in the hearts of creation’
- Transcribed from: Diseases of The Hearts & Their Cures | Shaykhul-Islaam Ibn Taymiyyah