Independent minds exploiting the methods of ijtihad sought to investigate the doctrines of the Quran, which until then had been accepted in blind faith on the authority of divine revelation. The first independent protest was that of the Kadar (Arabic: kadara, to have power), whose partisans affirmed the freedom of the will, in contrast with the Jabarites (jabar, force, constraint), who maintained the belief in fatalism.
In the second century of the Hegira, a schism arose in the theological schools of Bassora. A pupil, Wasil ibn Atha, who was expelled from the school because his answers were contrary to then orthodox Islamic tradition, proclaimed himself leader of a new school, and systematized the radical opinions of preceding sects, particularly those of the Kadarites. This new school or sect was called Mutazilite or Motazilite (from itazala, to separate oneself, to dissent). Its principal dogmas were three:
From the ninth century onward, owing to Calif al-Ma'mun and his successor, Greek philosophy was introduced among the Arabs, and the Peripatetic school began to find able representatives among them; such were Al-Kindi, Al-Farabi, Ibn Sina, and Ibn Roshd, all of whose fundamental principles were considered as heresies by the Motekallamin.
Aristotle attempted to demonstrate the unity of God; but from the view which he maintained, that matter was eternal, it followed that God could not be the Creator of the world. To assert that God's knowledge extends only to the general laws of the universe, and not to individual and accidental things, is tantamount to denying prophecy. One other point shocked the faith of the Motekallamin — the theory of the intellect. The Peripatetics taught that the human soul was only an aptitude — a faculty capable of attaining every variety of passive perfection — and that through information and virtue it became qualified for union with the active intellect, which latter emanates from God. To admit this theory would be to deny the immortality of the soul.
Wherefore the Motekallamin had, before anything else, to establish a system of philosophy to demonstrate the creation of matter, and they adopted to that end the theory of atoms as enunciated by Democritus. They taught that atoms possess neither quantity nor extension. Originally atoms were created by God, and are created now as occasion seems to require. Bodies come into existence or die, through the aggregation or the sunderance of these atoms. But this theory did not remove the objections of philosophy to a creation of matter.
For, indeed, if it be supposed that God commenced His work at a certain definite time by His "will," and for a certain definite object, it must be admitted that He was imperfect before accomplishing His will, or before attaining His object. In order to obviate this difficulty, the Motekallamin extended their theory of the atoms to Time, and claimed that just as Space is constituted of atoms and vacuum, Time, likewise, is constituted of small indivisible moments. The creation of the world once established, it was an easy matter for them to demonstrate the existence of a Creator, and that God is unique, omnipotent, and omniscient.
The oldest religio-philosophical work preserved is that of the Jewish philosopher Saadia Gaon (892-942), Emunot ve-Deot, "The Book of Beliefs and Opinions". In this work Saadia treats the questions that interested the Motekallamim, such as the creation of matter, the unity of God, the divine attributes, the soul, etc. Saadia criticizes other philosophers severely. For Saadia there was no problem as to creation: God created the world ex nihilo, just as the Bible attests; and he contests the theory of the Motekallamin in reference to atoms, which theory, he declares, is just as contrary to reason and religion as the theory of the philosophers professing the eternity of matter.
To prove the unity of God, Saadia uses the demonstrations of the Motekallamin. Only the attributes of essence (sifat-al-datiat) can be ascribed to God, but not the attributes of action (sifat-al-af'aliyat). The soul is a substance more delicate even than that of the celestial spheres. Here Saadia controverts the Motekallamin, who considered the soul an "accident" (compare "Moreh," i. 74), and employs the following one of their premises to justify his position: "Only a substance can be the substratum of an accident" (that is, of a non-essential property of things). Saadia argues: "If the soul be an accident only, it can itself have no such accidents as wisdom, joy, love," etc. Saadia was thus in every way a supporter of the Kalam; and if at times he deviated from its doctrines, it was owing to his religious views; just as the Jewish and Moslem Peripatetics stopped short in their respective Aristotelianism whenever there was danger of wounding orthodox religion.
The twelfth century saw the apotheosis of pure philosophy and the decline of the Kalam, which latter, being attacked by both the philosophers and the orthodox, perished for lack of champions. This supreme exaltation of philosophy was due, in great measure, to Gazzali (1005-1111) among the Arabs, and to Judah ha-Levi (1140) among the Jews. In fact, the attacks directed against the philosophers by Gazzali in his work, "Tu?fat al-Falasafa" (The Destruction of the Philosophers), not only produced, by reaction, a current favorable to philosophy, but induced the philosophers themselves to profit by his criticism, they thereafter making their theories clearer and their logic closer. The influence of this reaction brought forth the two greatest philosophers that the Arabic Peripatetic school ever produced, namely, Ibn Baja (Aven Pace) and Ibn Roshd (Averroes), both of whom undertook the defense of philosophy.
Since no idea and no literary or philosophical movement ever germinated on Arabian soil without leaving its impress on the Jews, Gazzali found an imitator in the person of Judah ha-Levi. This poet took upon himself to free his religion from what he saw as the shackles of speculative philosophy, and to this end wrote the "Kuzari," in which he sought to discredit all schools of philosophy alike. He passes severe censure upon the Motekallamin for seeking to support religion by philosophy. He says, "I consider him to have attained the highest degree of perfection who is convinced of religious truths without having scrutinized them and reasoned over them" ("Kuzari," v.). Then he reduced the chief propositions of the Motekallamin, to prove the unity of God, to ten in number, describing them at length, and concluding in these terms: "Does the Kalam give us more information concerning God and His attributes than the prophet did?" (Ib. iii. and iv.) Aristotelianism finds no favor in Judah ha-Levi's eyes, for it is no less given to details and criticism; Neoplatonism alone suited him somewhat, owing to its appeal to his poetic temperament.
Ibn Rushd (or Ibn Roshd or Averroes), the contemporary of Maimonides, closed the first great philosophical era of the Arabs. The boldness of this great commentator of Aristotle aroused the full fury of the orthodox, who, in their zeal, attacked all philosophers indiscriminately, and had all philosophical writings committed to the flames. The theories of Ibn Roshd do not differ fundamentally from those of Ibn Baja and Ibn Tufail, who only follow the teachings of Ibn Sina and Al-Farabi. Like all Arabic Peripatetics, Ibn Roshd admits the hypothesis of the intelligence of the spheres and the hypothesis of universal emanation, through which motion is communicated from place to place to all parts of the universe as far as the supreme world—hypotheses which, in the mind of the Arabic philosophers, did away with the dualism involved in Aristotle's doctrine of pure energy and eternal matter.
But while Al-Farabi, Ibn Sina, and other Arab philosophers hurried, so to speak, over subjects that trenched on religious dogmas, Ibn Roshd delighted in dwelling upon them with full particularity and stress. Thus he says, "Not only is matter eternal, but form is potentially inherent in matter; otherwise, it were a creation ex nihilo (Munk, "Mélanges," p. 444). According to this theory,therefore, the existence of this world is not only a possibility, as Ibn Sina declared—in order to make concessions to the orthodox—but also a necessity.
Driven from the Arabian schools, Arabic philosophy found a refuge with the Jews, to whom belongs the honor of having transmitted it to the Christian world. A series of eminent men—such as the Tibbons, Narboni, Gersonides—joined in translating the Arabic philosophical works into Hebrew and commenting upon them. The works of Ibn Roshd especially became the subject of their study, due in great measure to Maimonides, who, in a letter addressed to his pupil Joseph ibn Aknin, spoke in the highest terms of Ibn Roshd's commentary.
Modern Islamic philosophy seeks in some respects to renew the dialogue between Mutazilite and Asharite views about ethics in knowledge. An example is the Islamization of knowledge, and the view of khalifa of Seyyed Hossein Nasr. There is a separate article on these new trends.