Avicennism is a Medieval school of philosophy founded by the 11th Century Persian philosopher Avicenna (also known as Ibn Sina). Avicenna tried to redefine the course of Islamic philosophy and channel it into new directions, and particularly to reconcile Aristotelianism and Neo-Platonism with Islamic theology.
Avicenna's work, particularly his Metaphysics, had a profound influence on other medieval Scholastics such as St. Thomas Aquinas, Albertus Magnus and William of Auvergne. Despite some criticism by later Muslim theologians, Avicennism became the leading school of Islamic philosophy by the 12th Century, and even today forms the basis of philosophic education in the Islamic world
Early Islamic philosophy and theology distinguishes more clearly than Aristotelianism the difference between existence (the domain of the contingent and the accidental) and essence (which endures within a being, beyond the accidental). Avicenna argued that the fact of existence can not be inferred from, or accounted for, by the essence of existing things, and that form and matter by themselves cannot interact and originate the movement. He argued that some existing thing must necessitate, impart, give or add existence to an essence, and that "essence precedes existence" (Essentialism).
According to Avicenna, the universe consists of a chain of actual beings, each giving existence to, and responsible for, the rest of the chain below (angels, souls and all of creation). He argued that, as an infinite chain is impossible, the chain as a whole must terminate in a being that is wholly simple, self-sufficient and one, whose essence is its very existence (i.e. God). This is a combination of the Ontological Argument and Cosmological Argument for the existence of God (see the section on Philosophy of Religion), and a very early use of the method of a priori proof, utilizing intuition and reason alone.
Avicenna also developed his own system of Logic, known as Avicennian Logic, as an alternative to Aristotelian Logic, and by the 12th Century it had replaced Aristotelian Logic as the dominant system of Logic in the Islamic world. Avicennian Logic had an influence on early medieval European logicians such as Albertus Magnus, although Aristotelian Logic later became popular in Europe due to the strong influence of Averroism. Avicenna developed an early theory of the hypothetical syllogism as well as propositional calculus, an area of Logic not covered in the Aristotelianism tradition. He also contributed inventively to the development of inductive logic, mainly through his medical writings.
In Epistemology and the theory of knowledge, Avicenna developed the concepts of Empiricism and the tabula rasa (the idea that individual human beings are born with no innate or built-in mental content), which strongly influenced John Locke's formulation of tabula rasa and intuitive reasoning, and later gave rise to the nature versus nurture debate in modern philosophy and psychology. He was also the first to describe the methods of agreement, difference and concomitant variation which are critical to inductive logic and the scientific method, which was essential to later scientific methodology.
Later in the 12th Century, the Sufi mystic Shahab al-Din Suhrawardi (1155 - 1191) developed Illuminationism, a combination of Avicennism and ancient Persian philosophy, along with many new innovative ideas of his own. However, Avicennism was also criticized by several Muslim theologians.
Al-Ghazali (1058 - 1111), Fakhr al-Din al-Razi (1149 - 1209) and the Ash'ari theologians objected to Avicennism mainly on the grounds of its inconsistencies with the Qur'an and Hadith. Al-Ghazali's famous work "The Incoherence of the Philosophers" was specifically aimed at Avicenna, particularly his assertions that the world has no beginning in the past and is not created in time, that God's knowledge includes only classes of beings and not individual beings (universals not particulars), and that after death the souls of humans will never again return into bodies.
Averroės criticized Avicenna mainly due to his divergence from Aristotle. In particular, he rejected the theory of the celestial Souls and of an imagination which is independent of the corporeal senses. Averroism eventually proved more influential in the Christian West than Avicennism.
In the 17th Century, Mulla Sadra (c. 1571 1640) combined the vision of Sufi metaphysics with some of the rationalistic approach of Avicenna, eventually leading to a whole new philosophy known as Transcendent Theosophy. However, he opposed Avicennism's Essentialism, and espoused the opposite idea of "existence precedes essence", a key foundational concept of later Existentialism.