The theory of moderation is one of the common ethical theories among Muslim philosophers. According to this theory, all ethical virtues enjoy middle term, and all moral vices are rooted in going to extremes. Although the scholars of Islamic ethics were influenced by Plato and Aristotle in posing their theory of moderation, they also presented several innovations in this regard. Khwajah Nasir al-Din Tusi can be considered one of the innovators of this theory. His innovation in psychology consists of the four-fold classification of the faculties of the soul and designating justice as a virtue and as a function of the practical wisdom. However, his important contribution concerning the theory of moderation paved the way for presenting a new interpretation of the principle of moderation, adding the criterion of quality to it, and defining the concept of “malignity”. His other important innovation is determining three supreme genera of excess, neglect, and malignity for the vice, which, given the three faculties of the soul, will increase to nine in the number of supreme genera. It is worth mentioning that these novel contributions of Tusi greatly affected his succeeding philosophers.