Soon after al-Ghazâlî had published his two refutations of falsafa and Ismâ’îlism he left his position at the Nizâmiyya madrasa in Baghdad. During this period he began writing what most Muslim scholars regard as his major work, The Revival of the Religious Sciences (Ihyâ’ ‘ulûm al-dîn). The voluminous Revival is a comprehensive guide to ethical behavior in the everyday life of Muslims. It is divided into four sections, each containing ten books. The first section deals with ritual practices (‘ibâdât), the second with social customs (‘âdât), the third with those things that lead to perdition (muhlikât) and hence should be avoided, and the fourth with those that lead to salvation (munjiyât) and should be sought. In the forty books of the Revival al-Ghazâlî severely criticizes the coveting of worldly matters and reminds his readers that human life is a path towards Judgment Day and the reward or punishment gained through it. Compared with the eternity of the next life, this life is almost insignificant, yet it seals our fate in the world to come. In his autobiography al-Ghazâlî writes that reading Sufi literature made him realize that our theological convictions are by themselves irrelevant for gaining redemption in the afterlife. Not our good beliefs or intentions count; only our good and virtuous actions will determine our life in the world to come. This insight prompted al-Ghazâlî to change his lifestyle and adopt the Sufi path (al-Ghazâlî 1959a, 35–38 = 2000b, 77–80). In the Revival he composed a book about human actions (mu’âmalât) that wishes to steer clear of any deeper discussion of theological insights (mukâshafât). Rather, it aims at guiding people towards ethical behavior that God will reward in this world and the next (al-Ghazâlî 1937–38, 1:4–5).

In the Revival al-Ghazâlî attacks his colleagues in Muslim scholarship, questioning their intellectual capacities and independence as well as their commitment to gaining reward in the world to come. This increased moral consciousness brings al-Ghazâlî close to Sufi attitudes, which have a profound influence on his subsequent works such as The Niche of Lights (Mishkât al-anwâr). These later works also reveal a significant philosophical influence on al-Ghazâlî. In the Revival he teaches ethics that are based on the development of character traits (singl., khulq, pl. akhlâq). Performing praiseworthy deeds is an effect of praiseworthy character traits that warrant salvation in the next life (al-Ghazâlî 1937–38, 1:34.4–5). He criticizes the more traditional concept of Sunni ethics that is limited to compliance with the ordinances of the religious law (sharî’a) and following the example of the Prophet Muhammad. Traditional Sunni ethics are closely linked to jurisprudence (fiqh) and limit itself, according to al-Ghazâlî, to determining and teaching the rules of sharî’a. Traditional Sunni jurisprudents are mere “scholars of this world” (‘ulamâ’ al-dunyâ) who cannot guide Muslims on the best way to gain the afterlife (al-Ghazâlî 1937–38, 1:30–38, 98–140).

In his own ethics al-Ghazâlî stresses that the Prophet—and no other teacher—should be the one person a Muslim emulates. He supplements this key Sunni notion with the concept of “disciplining the soul” (riyâdat al-nafs). At birth the essence of the human is deficient and ignoble and only strict efforts and patient treatment can lead it towards developing virtuous character traits (al-Ghazâlî 1937–38, book 23). The human soul's temperament, for instance, becomes imbalanced through the influence of other people and needs to undergo constant disciplining (riyâda) and training (tarbiya) in order to keep these character traits at equilibrium. Behind this kind of ethics stands the Aristotelian notion of entelechy: humans have a natural potential to develop rationality and through it acquire virtuous character. Education, literature, religion, and politics should help realizing this potential. Al-Ghazâlî became acquainted with an ethic that focuses on the development of virtuous character traits through the works of Muslim falâsifa like Miskawayh (d. 1030) and Muslim scholars like al-Râghib al-Isfahânî (d. c.1025), who strove to make philosophical notions compatible with Muslim religious scholarship (Madelung 1974). As a result al-Ghazâlî rejected the notion, for instance, that one should try to give up potentially harmful affections like anger or sexual desire. These character traits are part of human nature, al-Ghazâlî teaches, and cannot be given up. Rather, disciplining the soul means controlling these potentially harmful traits through one's rationality (‘aql). The human soul has to undergo constant training and needs to be disciplined similar to a young horse that needs to be broken in, schooled, and treated well.

At no point does al-Ghazâlî reveal the philosophical origins of his ethics. He himself saw a close connection between the ethics of the falâsifa and Sufi notions of an ascetic and virtuous lifestyle. In his Revival he merges these two ethical traditions to a successful and influential fusion. In his autobiography al-Ghazâlî says that the ethics of the falâsifa and that of the Sufis are one and the same. Congruent with his position that many teachings and arguments of the falâsifa are taken from earlier revelations and from the divinely inspired insights of mystics, who existed already in pre-Islamic religions (Treiger 2012, 99–101) he adds that the falâsifa have taken their ethics from the Sufis, meaning here mystics among the earlier religions (al-Ghazâlî 1959a, 24 = 2000b, 67).

Another important field where al-Ghazâlî introduced Avicennan ideas into Ash'arite kalâm in a way that this tradition eventually adopted them is human psychology and the rational explanation of prophecy (Griffel 2004, al-Akiti 2004). Based on partly mis-translated texts by Aristotle (Hansberger 2011), Avicenna developed a psychology that assumes the existence of several distinct faculties of the soul. These faculties are stronger or weaker in individual humans. Prophecy is the combination of three faculties which the prophet has in an extraordinarily strong measure. These faculties firstly allow the prophet to acquire theoretical knowledge instantly without learning, secondly represent this knowledge through symbols and parables as well as divine future events, and thirdly to bring about effects outside of his body such as rain or earthquakes. These three faculties exist in every human in a small measure, a fact proven by the experience of déjà vu, for instance, a phenomenon referred to in the Arabic philosophic tradition as “the veridical dream” (al-manâm al-sâdiq). Al-Ghazâlî adopted these teachings and appropriated them for his own purposes (Treiger 2012). The existence of the three faculties in human souls that make up prophecy serves for him as an explanation of the higher insights that mystics such as Sufi masters have in comparison to other people. While prophets have strong prophetic faculties and ordinary humans very weak ones, the “friends of God” (awliyâ', i.e. Sufi masters) stand in between these two. They are endowed with “inspiration” (ilhâm), which is similar to prophecy and which serves in al-Ghazâlî as one of the most important sources of human knoweldge. Unlike Avicenna, for whom prophets and maybe also some particularly talented humans ('ârifûn in his language) acquire the same knowledge that philosophers reach through apodictic reasoning, in al-Ghazâlî the prophets and awliyâ' have access to knowledge that is superior to that available solely through reason.

Despite the significant philosophical influence on al-Ghazâlî's ethics, he maintained in Islamic law (fiqh) the anti-rationalist Ash’arite position that human rationality is mute with regard to normative judgments about human actions and cannot decide whether an action is “good” or “bad.” When humans think they know, for instance, that lying is bad, their judgment is determined by a consideration of their benefits. With regard to the ethical value of our actions we have a tendency to confuse moral value with benefit. We generally tend to assume that whatever benefits our collective interest is morally good, while whatever harms us collectively is bad. These judgments, however, are ultimately fallacious and cannot be the basis of jurisprudence (fiqh). “Good” actions are those that are rewarded in the afterlife and “bad” actions are those that are punished (al-Ghazâlî 1904–07, 1:61). The kind of connection between human actions and reward or punishment in the afterlife can only be learned from revelation (Hourani 1976, Marmura 1968–69). Muslim jurisprudence is the science that extracts general rules from revelation. Like most religious sciences it aims at advancing humans' prospect of redemption in the world to come. Therefore it must be based on the Qur’an and the sunna of the Prophet while it uses logic and other rational means to extract general rules.

Al-Ghazâlî was one of the first Muslim jurists who introduced the consideration of a “public benefit” (maslaha) into Muslim jurisprudence. In addition to developing clear guidance of how to gain redemption in the afterlife, religious law (sharî’a) also aims at creating an environment that allows each individual wellbeing and the pursuit of a virtuous and pious lifestyle. Al-Ghazâlî argues that when God revealed divine law (sharî’a) He did so with the purpose (maqsad) of advancing human benefits in this world and the next. Al-Ghazâlî identifies five essential components for wellbeing in this world: religion, life, intellect, offspring, and property. Whatever protects these “five necessities” (al-darûriyyât al-khamsa) is considered public benefit (maslaha) and should be advanced, while whatever harms them should be avoided. The jurisprudent (faqîh) should aim at safeguarding these five necessities in his legal judgments. In recommending this, al-Ghazâlî practically implies that a “maslaha mursala,” a public benefit that is not mentioned in the revealed text, is considered a valid source of legislation (Opwis 2007 and 2010, 65–88)