Avicenna (980-1037), Abdaallah Ibn Sina, known as "The Supreme Master," was the greatest of the Islamic thinkers. Born in Bukhara, Persia, he became physician and adviser to sultans and princes. His Canon of Medicine, written at the age of 21, was the best-known medical text in Europe and Asia for several centuries. He authored over a hundred works in medicine and philosophy that have inspired innumerable commentaries. His most important books in philosophy were The Healing (al-Shifa) and Demonstrations and Affirmations. He died in Hamadan in northern Persia.

Avicenna made enduring contributions to the areas under discussion here. He is especially famous for his insights into the necessary existence of God and the non-material nature of the human soul.
While physics is concerned with the motion of things, metaphysics focuses on the very existence of things. Why and how is it that they happen to exist? There is no scientific or logical law that says they must necessarily exist. There is only a possibility of their existing, and an equally real possibility that they might not have existed. But, unlike all other beings, God exists by necessity, and his non-existence is impossible. To exist belongs to the very essence or nature of God. He exists, and cannot not-exist.

The existence of beings that do not exist by an inner necessity of their natures points to the existence of the necessary being, God. Even an infinite chain of these beings that are each caused to exist by a source external to itself cannot explain how any or all of them came to exist. Only a first cause that exists necessarily can explain the existence of every other being.

"This is what it means that a thing is created, that is, receiving its existence from another," writes Avicenna, "As a result everything, in relation to the first cause, is created...Therefore, every single thing, except the primal One, exists after not having existed with respect to itself." That is, anything brought into existence by the first cause requires the action of this cause to remain in existence. Avicenna writes, "That which is caused requires something which bestows existence upon it continuously, as long as it continues as existing."

No cause is required to explain the existence of a necessarily existing being. Avicenna observes, "That whose existence is necessary through itself does not have a cause while that whose existence is possible through itself does have a cause." And there can be only one necessary being. "That whose existence is necessary must necessarily be one essence" is the first volley of his elaborate argument to prove this particular thesis.

Like other thinkers influenced by Aristotle and Plato, Avicenna maintained that there was a hierarchy of intelligent beings in the universe. This scheme led some critics to call him a pantheist. But these accusations are implausible, given that it was Avicenna who underscored the radical difference between God, the necessary being whose essence is to exist, and all other beings.

Although Avicenna believed that the world is a creation of God, he also believed, under the influence of Aristotle, that both God and the world existed eternally. As Aquinas and other theists acknowledge, this view is not self-contradictory because creation does not necessarily require a beginning in time. Avicenna, of course, noted that in itself the world is only "possible" and requires a cause for its existence. God, on the other hand, exists necessarily and brought the world into being from nothing. This act could either have a beginning or be beginningless and endless. Other Islamic philosophers put forward the kalam argument, made famous in recent times by William Lane Craig, which shows that the universe had to have a beginning in time. F.F. Centore observes that one defect of Avicenna's thought was his assumption that the world necessarily emanates from God.

Avicenna also introduced innovative arguments to show that the human soul is immaterial and indivisible. He noted that each person is ineradicably aware of his/her existence as an individual self, a self that will permanently retain its individuality.

To find out more about Avicenna and the "Theory of Everything" he shares in common with Maimonides, Aquinas and Mahdvacharya, read the book The Wonder of the World: A Journey from Modern Science to the Mind of God by Roy Abraham Varghese.