Ibn Sina proved himself to be a great physician early on in his career, and was appointed by kings and sultans throughout his life as their court physician. He discovered diabetes, tuberculosis, and cataract, among other achievements. His most influencial and encyclopedic book, The Canon of Medicine, remained the number one authority on medicine throughout Europe, North Africa, and Asia for over 700 years, taught to students at the University of Paris up until the 18th century.
But in addition to his fame in medicine, he was also famous during the Islamic golden age as one of the greatest Muslim philosophers, second only to al-Farabi. He had major contributions and writings in psychology, logic, mathematics, chemistry, astronomy, pharmacology, poetry, music, metaphysics, jurisprudence, engineering, and politics. He was also the head of an army brigade and was known for his bravery in combat.
The following are excerpts from his most famous writings.
On the Proof of Prophecies
I now say: there exists in man a faculty by which he is differentiated from the rest of animals and other things. This is called the rational soul. It is found in all men without exception, but not in all its particulars since its powers very among men. Thus there is a first power ready to become informed with the universal forms abstracted from matter, which in itself has no form. For this reason it is called the material intellect by analogy with prime matter....
The forms and materials that constitute bodies are either organic or inorganic, and the first are better. The organic are either animals or not animals, and the first are better. The animals, in turn, are either rational or irrational, and the first are better. The rational either possess reason by positive disposition or do not, and the first are better. That which is rational by positive disposition either becomes completely actual or does not, and the first is better. That which becomes completely actual does so without mediation or through mediation, and the first is better. This is the one called prophet and in him the degrees of excellence in the realm of material forms culminate....
Revelation is the emanation and the angel is the received emanating power that descends on the prophets as if it were an emanation continuous with the universal intellect....
As for the validity of the prophethood of our prophet, of Muhammad (may God's blessings and peace be on him), it becomes evident to the reasonable man once he compares him with the other prophets, peace be on them....
It has been said that a condition the prophet must adhere to is that his words should be symbols and his expressions hints. Or, as Plato states in the Laws: whoever does not understand the apostles' symbols will not attain the Divine Kingdom. Moreover, the foremost Greek philosophers and prophets made use in their books of symbols and signs in which they hid their secret doctrine - men like Pythagoras, Socrates, and Plato. As for Plato, he had blamed Aristotle for divulging wisdom and making knowledge manifest so that Aristotle had to reply: "Even though I had done this, I have still left in my books many a pitfall which only the initiate among the wise and learned can understand." Moreover, how could the prophet Muhammad (may God's prayers and peace be on him) bring knowledge to the uncouth nomad, not to say to the whole human race considering that he was sent a messenger to all? Political guidance, on the other hand, comes easily to prophets; also the imposition of obligations on people.
The first thing you asked me about was what the prophet Muhammad (may God's prayers and peace be on him) conveyed from his Lord (may He be honored and glorified), saying: God is the light of the heavens and the earth. The likeness of His light is as a niche wherein is a lamp (the lamp in a glass, the glass is as it were a brilliant star) kindled from a blessed tree, an olive neither from the east nor from the west. Its oil almost shines even if no fire touched it. Light upon light! God guides to His light whom He will. God strikes similitudes for men. God has knowledge of everything [xxiv, 35].
I say: light is an quivocal term partaking of two meanings, one essential, the other metaphorical. The essential stands for the perfection of the transparent in asmuch as it is transparent, as Aristotle said. The metaphorical meaning is to be understood in two ways: either as the good, or as the cause that leads to the good. Here, the sense is the metaphorical one in both meanings. I mean that God, the Exalted, is in Himself the good and the cause of everything good. The same judgment applies to the essential and to the nonessential. The heavens and the earth stands for the "whole." The niche stands for the material intellect and the rational soul. For the walls of a niche are close to each other and it is thus excellently predisposed to be illuminated since the closer the walls of a place are to each other, the greater the reflection and the light it holds. And just as the actualized intellect is likened to light, its recipient is likened to the recipient of light, which is the transparent. The best of transparent things is air and the best [transparent] air is in the niche. Thus what is symbolized by the niche is the material intellect, which is to the acquired intellect as the niche is to the light. The lamp stands for the acquired actualized intellect. For light, as the philosophers defined it, is the perfection of the transparent and that which moves it from potentiality to actuality. The acquired intellect is to the material intellect as the lamp is to the niche.
The expression in a glass is used because between the material and the acquired intellects there exists another [intermediate] level or place that is related to these two as that which intervenes between what is transparent and the lamp is related to the latter two. Here, in a visual sight, the lamp does not reach [and hence could not be seen through] the transparent [air] without a medium. This is the oil vessel with the wick, from which the glass protrudes. For glass is one of the transparent things receptive of light. Hence the subsequent utterance, is as it were a brilliant star, is given to convey that it is pure transparent glass, not opaque colored glass, since nothing colored is transparent. By kindled from a blessed tree, an olive, is meant the cogitative power, which stands as subject and material for the intellectual acts in the same way that oil stands as subject and the material for the lamp.
Neither from the east nor from the west is explained as follows: "East" lexically derives from the place whence light emanates and "west" where it is quenched; and east is used metaphorically for the place where there is light and west for the place where there is no light. (Notice how the rules of simile are adhered to: light was made the basis of the statement and the simile constructed thereon; light was conjoined with the apparatus and materials that produce it.) Thus what is symbolized by the expression, neither from the east nor from the west, is as follows: the cogitative power, in the absolute sense, is not one of the pure rational powers where light emanates without restriction. This is the meaning of the saying, a ... tree ... neither from the east. Nor is it on eof the animal powers where light is utterly lost. This is the meaning of nor from the west.
The saying, its oil almost shines even if no fire touched it, is in praise of the cogitative power. In this expression, even if no fire touched it, the word touch stands for connection and emanation. The saying fire is explained as follows: when the similarity between metaphorical light and real light and between the instruments and the consequences of the former and those of the latter was drawn, the essential subject that causes a thing to be in another was likened to what is customarily considered a subject, that is to say, fire. For although in reality fire is colorless, custom takes it to be luminous. (Observe how the rules of simile are adhered to!) Moreover, since fire surrounds elements (ummahat), that which surrounds the world, not in the spatial sense, but in a verbal metaphorical sense, is likened to fire. This is the universal intellect. This intellect, however, is not as ALexander of Aphrodisias believed - attributing the belief to Aristotle - the true God, the First. For although in one respect this first intellect is one, it is multiple inasmuch as it consists of the forms of numerous universals. It is thus one, not essentially, but accidentally, acquiring its oneness from Him who is essentially one, the one God (may He be magnified).
Translated by Michael E. Marmura
Without doubt, the fundamental principle here is that men must continue in their knowledge of God and the resurrection and that the cause of forgetting these things with the passage of the generation succeeding [the mission of] the prophet (may God's prayers and peace be on him) must be absolutely eliminated. Hence there must be certain acts and works incumbent on people that the legislator must prescribe to be repeated at frequent specified intervals. In this way memory of the act is renewed and reappears before it can die.
These acts must be combined with what brings God and the afterlife necessarily to mind; otherwise they are useless.... Now reminders consist of either motions or the absence of motions that lead to other motions. An example of motion is prayer; of the absence of motion, fasting. For although the latter is a negative motion, it so greatly moves one's nature that he who fasts is reminded that what he is engaged in is not a jest. He will thus recall the intention of his fasting, which is to draw him close to God.
The noblest of these acts of worship, from one point of view, should be the one in which the worshiper considers himself to be addressing God, beseeching Him, drawing close to Him, and standing in His presence. This is prayer. [...]
These acts benefit the worshipers in that they perpetuate in the latter adherence to the laws and religion (shari'ah) that insure their existence and in that, by virtue of the goodness they inspire, they bring the worshipers closer to God in the hereafter.
Translated by Michael E. Marmura