Ibn Sina

How to use this course

Ibn Sina, known in Latin as Avicenna, is remembered as a metaphysician and as the author of the Canon of Medicine, which was taught in European universities for five centuries. This course looks at a less familiar side: the natural philosopher. In the physics of his great encyclopedia, Kitab al-Shifaʾ (The Cure), he reworked Aristotle’s account of motion, gave a new explanation of why thrown bodies keep moving, argued about void and indivisible atoms, and set out how a science should establish its principles. The same habits of mind organize his medicine.

Each technical lesson opens with a paragraph for the general reader, then states Ibn Sina’s positions as precisely as the texts allow, reconstructs his arguments and compares them with modern physics. Formulas are modern; Ibn Sina wrote in words. When we use symbols to model his claims, we say so. Exercises have folded solutions. The biography rests mainly on his own autobiography, continued after his death by his pupil al-Juzjani; we mark where other sources or later stories enter.

Timeline

Bukhara, c. 980–999

Abu ʿAli al-Husayn ibn ʿAbd Allah ibn Sina was born at Afshana, a village near Bukhara, in today’s Uzbekistan. Bukhara was the capital of the Samanids, a Persian dynasty that ruled Transoxiana and Khorasan nominally on behalf of the Abbasid caliph. His father ʿAbd Allah came from Balkh and served as a Samanid official, governing a village on a royal estate. His mother was from Afshana. The family moved to Bukhara when he was young.

The autobiography

Our main source is a short autobiography that Ibn Sina dictated to his pupil Abu ʿUbayd al-Juzjani, who added an account of his master’s later life. It is a self-portrait with a purpose: Ibn Sina presents himself as a largely self-taught student who reached all the sciences by his own intelligence. Historians accept its main facts and read its tone with care.

By his own account he had learned the Quran and much literature by about ten. He studied arithmetic with a greengrocer who used Indian calculation and law with the Hanafi jurist Ismaʿil al-Zahid. His father’s household discussed the doctrines of the Ismaʿili missionaries, which he listened to without accepting. A visiting teacher, Abu ʿAbd Allah al-Natili, began him on logic with Porphyry’s Isagoge; the pupil soon outran the teacher and worked through Euclid and Ptolemy’s Almagest on his own.

Medicine and metaphysics

He took up medicine in his early teens, found it easy compared with mathematics and metaphysics, and was treating patients by about sixteen. The hard subject was metaphysics. He says he read Aristotle’s Metaphysics forty times, until he knew it by heart, without grasping its purpose. Then a bookseller pressed on him a cheap copy of al-Farabi’s short treatise on the aims of the Metaphysics, and the meaning opened at once. He gave alms to the poor in thanks.

At about seventeen he was called to treat the Samanid ruler Nuh II ibn Mansur, and as a reward was allowed into the royal library. He describes rooms of chests, each for one science, and books whose names most people had never heard. He wrote his first works, on the soul and on philosophy, before he was twenty-two. Soon after, his father died and the Samanid state fell: the Qarakhanids took Bukhara in 999.

Wandering years, 999–1015

Without the Samanids, Ibn Sina needed new patrons. For the rest of his life he moved among the small dynasties of Central Asia and Iran, serving as physician, counsellor and resident philosopher. Al-Juzjani says “necessity” drove him from one court to the next.

Gurganj

He went first to Gurganj, the capital of Khwarazm, under the Maʾmunid ruler Abu’l-Hasan ʿAli. The Maʾmunid court also patronized the astronomer Abu Rayhan al-Biruni. The two men exchanged a famous set of questions and answers. Al-Biruni asked eighteen questions on physics and cosmology, ten of them aimed at Aristotle’s On the Heavens. Ibn Sina answered for the Aristotelians, and al-Biruni replied in turn. The exchange shows that the questions treated in Lessons 04 to 08 were alive in the eastern courts.

Jurjan and Rayy

As the power of the Ghaznavid ruler Mahmud pressed on Khwarazm, Ibn Sina left. He hoped for the patronage of Qabus, the scholarly emir of Jurjan near the Caspian, but Qabus had been deposed and died. At Jurjan in 1012–13 he met Abu ʿUbayd al-Juzjani, who stayed with him until his death. There he began the Canon of Medicine. Around 1014 he served Majd al-Dawla and his mother Sayyida at Rayy, near modern Tehran, then moved on to Qazvin and Hamadan.

Vizier and prisoner in Hamadan, c. 1015–1023

At Hamadan Ibn Sina treated the Buyid emir Shams al-Dawla and became his court physician, then his vizier. The army mutinied against him, demanded his execution and plundered his house; he hid, was recalled when the emir fell ill again, and served a second term as vizier.

Writing by night

Al-Juzjani describes the working routine. By day Ibn Sina served the state. At night students gathered at his house to read the Shifaʾ and the Canon as they were written, with wine and music when the reading was done. Al-Juzjani had asked him to write commentaries on Aristotle; Ibn Sina declined and instead wrote a summa that would cover all of philosophy in his own order. That became the Shifaʾ.

Hiding and prison

Shams al-Dawla died in 1021. His son wanted to keep Ibn Sina as vizier, but Ibn Sina secretly corresponded with ʿAlaʾ al-Dawla, ruler of Isfahan. He hid in the house of a druggist, Abu Ghalib al-ʿAttar, and there, al-Juzjani says, wrote out the headings of the whole Shifaʾ and then filled them in rapidly without consulting books, completing most of the natural philosophy and metaphysics. When his correspondence was discovered, he was imprisoned for about four months in the fortress of Fardajan near Hamadan. According to al-Juzjani he kept writing in prison; one work from this period is the allegory Hayy ibn Yaqzan (Alive, son of Awake). He later escaped to Isfahan disguised as a Sufi, with his brother, al-Juzjani and two slaves.

Nature, motion and time

For the general reader. For Aristotle and his followers, physics was the study of things that change. Motion meant any gradual change: moving from place to place, growing, heating up. Ibn Sina kept this framework but tightened it, asking what exactly exists at an instant while a body is moving, and adding a new kind of motion to explain spinning bodies.

The plan of the Shifaʾ

The Shifaʾ has four parts: logic, natural philosophy, mathematics (geometry, arithmetic, music, astronomy) and metaphysics. The natural philosophy opens with the Physics (al-Samaʿ al-tabiʿi), which studies body insofar as it is subject to motion, and continues with books on the heavens, generation and corruption, actions and passions, meteorology, minerals, the soul, plants and animals. Natural science takes its principles, such as the existence of nature and of motion, from metaphysics or from experience, and does not prove them.

What motion is

A body that can be in another place is, while moving, partly realized: it is actually on its way. Ibn Sina then distinguished two things the word can mean.

• Traversal motion (al-haraka al-qatʿiyya): the whole extended passage from start to finish. It never exists all at once, so it exists only in the mind, which joins the earlier and later positions.
• Medial motion (al-haraka al-tawassutiyya): the state of being between start and finish, which exists in the body at each instant of the motion.

The distinction answers an old puzzle: at an instant a body occupies one place, so where is the motion? Ibn Sina’s answer is that something real, being-in-transit, is present at the instant, though the extended path is not. A modern physicist would say the instantaneous velocity is a real property of the state.

Aristotle allowed motion in three categories: quantity, quality and place. Ibn Sina added a fourth, motion in position (wadʿ), for a sphere that rotates without changing its place as a whole. This made the rotation of the celestial spheres a motion in its own right.

Time

Ibn Sina argued for the reality of time from a capacity. Two bodies starting together at the same speed may stop together or at different moments; in the second case they cover different distances. There is thus a magnitude, the “possibility” for motion of a given speed to cover a given distance, that is neither the distance nor the motion. That magnitude is time. In modern terms, \(t=d/v\) for uniform motion, and the existence of the ratio is the argument.

Inclination: the theory of mayl

For the general reader. Why does a stone keep moving after it leaves your hand? Aristotle thought the air pushed it. Ibn Sina rejected this and said the thrower gives the stone an internal quality, an inclination, that carries it on until the air and the stone’s weight wear it away. In empty space nothing would wear it away and the stone would never stop. This is a step toward the idea of inertia, though Ibn Sina drew the opposite conclusion: since endless motion is impossible, empty space is impossible.

The problem of projectiles

In Aristotle’s physics every motion that is not natural needs a mover in contact with the moved. A heavy stone moves naturally downward. When thrown upward or sideways its motion is violent, and after release there seems to be no mover. Aristotle answered that the hand also moves the air, and the air carries the stone. The Alexandrian John Philoponus (6th century) objected and proposed that the thrower impresses an incorporeal power on the stone, which spends itself and fades.

Ibn Sina’s account in the Physics

Ibn Sina considered the two air theories. On the first, the air in front of the stone circles round and pushes it from behind; he found it unclear how this could work. On the second, the air set moving by the hand pushes the stone; he admitted that wind moves heavy things but found it implausible that air could carry a stone over a long throw. He concluded that the thrower imparts a violent mayl to the stone. The mayl is a quality distinct from the stone’s nature and is not used up by itself. It weakens only through the resistance of the medium and the opposing natural inclination of the body.

A modern model

The texts are qualitative. To compare them with Newton, let \(m\) be the mayl imparted horizontally, \(k\) a resistance coefficient of the medium, and suppose with Ibn Sina that the speed is proportional to the mayl. Then one simple model is

For \(k>0\) the motion fades; for \(k=0\) (void) it continues with constant speed forever. Newton’s first law gives the same conclusion for \(k=0\), and linear drag gives the same exponential law for \(k>0\). The differences lie elsewhere. Ibn Sina’s mayl is a cause that the motion requires at every moment, while Newtonian momentum is a state that needs no cause to persist. And Ibn Sina used the void case to refute void, not to describe nature.

After Ibn Sina

In the twelfth century Abu’l-Barakat al-Baghdadi held that the violent mayl weakens of itself, as Philoponus had, and explained the acceleration of falling bodies by successive increments of mayl. In fourteenth-century Paris, John Buridan named the imparted quality impetus, held that it does not decay of itself, and made it grow with the speed and the quantity of matter of the body. How directly Buridan drew on Ibn Sina is debated; Aydın Sayılı compared the two theories closely.

Void, the continuum and atoms

For the general reader. Is matter made of smallest pieces, or can it be divided forever? Is there empty space between things? Muslim theologians of the kalam held that bodies are made of indivisible atoms. Ibn Sina argued that matter is continuous and that there is no void, using some of the same geometric arguments that later philosophers would repeat.

Place

Some philosophers held that place is a three-dimensional extension, a space that bodies occupy and that could exist empty. Ibn Sina argued that such an empty extension would have to be something with a definite nature, but no positive description of it can be given. Measurable extension is a feature of bodies, so a measurable void would be a body. In addition he used the mayl argument of Lesson 05.

Against atoms

Kalam atomists held that bodies consist of finitely many indivisible parts. Ibn Sina gave physical and geometric objections. One concerns the diagonal.

Ibn Sina’s own example uses a right isosceles triangle with legs of three units, so the hypotenuse is \(\sqrt{18}\approx4.24\) units, which no whole count of atoms can match. The atomist must either deny the Pythagorean theorem or admit that atoms can be divided.

Potential division and natural minima

A continuous body has no actual parts, Ibn Sina said; parts exist only when we posit them, by cutting or by imagining. Division can go on without end, but it never completes an actual infinity. Yet he also held that a physical body cannot be divided indefinitely and keep its nature. A tiny drop of water in hot air cannot keep its form and becomes air. These natural minima are limits of physical substances, not of geometry.

Mechanics: weights, speeds, machines

For the general reader. In the medieval scheme of knowledge, the science of weights and machines belonged to mathematics, not to physics. Ibn Sina wrote on both sides: in the physics of the Shifaʾ he discussed how heavy bodies fall and how inclination grows near a body’s natural place; in a treatise on machines attributed to him he analysed the lever, pulley, windlass, wedge and screw.

Falling bodies

Natural mayl is the heaviness of a body, which is strongest when the body is out of its natural place and pushes it back. Ibn Sina held, following Aristotle, that a larger heavy body has more mayl and that natural motion speeds up as the body approaches its natural place. He did not give a quantitative law. The successors who took the idea further, Abu’l-Barakat al-Baghdadi and later the Parisian impetus theorists, explained acceleration as the accumulation of increments of mayl or impetus, which is close in spirit to \(\Delta v=g\,\Delta t\).

The simple machines

The Greek tradition of Heron of Alexandria recognized five simple machines: windlass, lever, pulley, wedge and screw. A treatise on machines, Miʿyar al-ʿuqul (The measure of the intellects), written in Persian and attributed to Ibn Sina, describes these machines, their combinations, and how each lets a small force move a large weight. Its authenticity has been questioned, so we treat its content as Avicennian tradition.

Every simple machine obeys the same principle in modern form: in an ideal machine the work put in equals the work got out. If the effort \(F\) moves through distance \(s_F\) while the load \(W\) rises through \(s_W\), then

• Windlass: handle radius \(R\), axle radius \(r\): \(W/F=R/r\).
• Pulley system with \(n\) strands supporting the movable block: \(W/F=n\).
• Wedge of length \(L\) and back \(t\): \(W/F=L/t\) ignoring friction.
• Screw of pitch \(p\) turned by a lever of radius \(R\): \(W/F=2\pi R/p\).

Light and vision

For the general reader. Do we see because something goes out from the eye, or because something comes in? Many Greek mathematicians thought the eye sends out rays. Ibn Sina argued at length that nothing leaves the eye: the illuminated transparent air carries the visible form of the object to the eye, which receives it as a mirror receives an image. He reached this conclusion independently of his contemporary Ibn al-Haytham, whose experimental optics went much further.

Three theories

• Extramission (Euclid, Ptolemy): rays go out from the eye in a cone and touch the object.
• Emanation of copies (the atomists): thin films peel off the object and enter the eye.
• Aristotle’s medium theory: light makes a transparent medium actually transparent, and the object’s colour then alters the medium all the way to the eye.

Ibn Sina, in the psychology of the Shifaʾ and in shorter works, sided with Aristotle. Against extramission he argued that a ray leaving the eye would have to be a body, and a body large enough to fill half the visible sky would have to come out of a small eye at once. If the ray were not a body, it could not carry anything back. He used a geometric diagram of the arcs an object subtends at the eye at different distances to explain why far things look small, without any outgoing rays. The eye, he said, is like a mirror: the object is reflected in it, and if a mirror had a soul it would see.

Light, speed and the rainbow

Ibn Sina held that if seeing depends on something travelling from the luminous source, that travel must take time, so the speed of light would be finite. He also distinguished images that stay fixed on a surface, such as green light from a lawn falling on a wall, from images that move with the observer, as in a mirror. His account of the rainbow in the Meteorology placed the bow in a thin mist rather than in the dark cloud and treated its colours as arising in the eye; he admitted he was not satisfied with it, and it does not use refraction in drops. The correct explanation came from Kamal al-Din al-Farisi and Theodoric of Freiberg around 1300 and was completed by Descartes.

Relation to Ibn al-Haytham

Ibn al-Haytham, working in Cairo in the same decades, also rejected extramission, and combined intromission with a geometry of rays from each point of the object, tested by experiment. David Lindberg found no convincing evidence that either man read the other; their agreements come from shared sources.

Logic: the instrument of science

For the general reader. Logic, for Ibn Sina, is the tool that keeps reasoning from error. He rebuilt Aristotle’s logic so thoroughly that later logicians in the Islamic world taught his system instead of Aristotle’s. He noticed that ordinary statements hide conditions of time (“every moving thing changes” is true only while it moves), and he built a logic of such conditions.

Conception and assent

All knowledge, Ibn Sina said, is either tasawwur (conception: grasping what a thing is, reached by definition) or tasdiq (assent: judging that something is true, reached by syllogism). Logic has two branches accordingly. Its subject matter is the secondary intelligibles: concepts such as genus, species, proposition and syllogism, which apply to other concepts rather than directly to things.

Two readings of a proposition

“Every moving thing changes” is true on the descriptional reading and false on the essential one, since a moving thing need not change when it stops. Ibn Sina also replaced Aristotle’s plain assertoric proposition with the absolute (mutlaqa) proposition, read as “at some time”: “every \(J\) is \(B\) at least once”. Its contradictory is then not another absolute proposition but a perpetual one: “some \(J\) is never \(B\)”.

Syllogisms

• Connective (iqtirani) syllogisms link premises through a shared term, as in Barbara: every \(B\) is \(A\), every \(C\) is \(B\), so every \(C\) is \(A\). Ibn Sina extended them to conditional premises: always if \(p\) then \(q\); always if \(q\) then \(r\); so always if \(p\) then \(r\).
• Repetitive (istithnaʾi) syllogisms repeat one part of a conditional or disjunctive premise: if \(p\) then \(q\); but \(p\); so \(q\) (modus ponens), or \(p\) or \(q\); but not \(p\); so \(q\).

He distinguished three disjunctions: real (exactly one holds), one that forbids both being true, and one that forbids both being false.

Experience as a source of principles

In the Burhan (Demonstration) of the Shifaʾ, Ibn Sina asked how the premises of a science are known. Induction from cases cannot give certainty, because unobserved cases may differ. Experience (tajriba) can, when it is controlled: we observe repeatedly that scammony purges bile, and add the implicit syllogism that a regular effect cannot come about by chance, so scammony has a power to purge bile. The conclusion holds with a qualification: scammony purges bile in our lands and in normal conditions, until a contrary instance shows a hidden factor.

Medicine as a science

For the general reader. The Canon of Medicine organized the whole of Greek and Arabic medicine into one system: general theory, drugs, diseases from head to foot, whole-body diseases and compound remedies. Ibn Sina treated medicine as a science with principles taken from natural philosophy, and he set out rules for testing whether a drug really works that read today like rules for controlled experiments.

The structure of the Canon

1. General principles: elements, temperaments, humours, anatomy, physiology, causes and signs of disease, general therapy.
2. About 800 simple drugs, arranged alphabetically.
3. Diseases of particular organs, from head to foot.
4. Diseases not specific to one organ, such as fevers and poisons.
5. A formulary of about 650 compound drugs.

Book I opens by defining medicine as the science of the states of the human body with respect to health and its loss, so as to preserve the one and restore the other. Its principles come from natural philosophy, as the principles of every subordinate science come from a higher one. The causes of health and disease are arranged under Aristotle’s four causes: the material (the body and its humours), the efficient (air, food, sleep, emotions and other agents), the formal (the temperament or constitution), and the final (the functions).

Rules for testing a drug

The rules are the logic of tajriba (Lesson 09) applied to drugs. Rules 1 and 2 control confounding variables; rule 4 is a dose–response study; rule 6 asks for reproducibility; rule 7 warns against extrapolating from animal models. What is missing is a comparison group and randomization, which entered medicine much later.

Contagion

The Canon recognizes the contagious nature of phthisis (tuberculosis), a point George Sarton singled out for praise. Ibn Sina explained contagion within the humoral framework, without a theory of germs.

Isfahan and the last years

At Isfahan the Kakuyid ruler ʿAlaʾ al-Dawla gave Ibn Sina a position at court and the company of scholars. Peter Adamson calls these final years, about thirteen of them, the most stable period of his life. He finished the Shifaʾ, wrote its abridgement the Najat (The Salvation), the Persian Danishnama-yi ʿAlaʾi (Book of Knowledge for ʿAlaʾ al-Dawla), and his last summa, al-Isharat wa’l-tanbihat (Pointers and Reminders).

He also did astronomy. ʿAlaʾ al-Dawla asked him to make observations and correct the tables; al-Juzjani says he did so and devised instruments. One, described in a short treatise, measured the altitude and azimuth of a star with sighted legs over a protractor. He wrote a summary of the Almagest. He reported seeing Venus as a spot on the face of the Sun, which, if true, would place Venus below the Sun; without a date the report cannot be checked, and modern scholars doubt it. In mineralogy he rejected the alchemists’ claim to transmute one metal into another, and he explained mountains by uplift and by slow erosion of rock layers of unequal hardness.

In 1030 the Ghaznavid army sacked Isfahan, and his book al-Insaf (Fair Judgment), which judged between Eastern and Western Aristotelians, was lost. In 1037, travelling with ʿAlaʾ al-Dawla toward Hamadan, he suffered a severe attack of colic. Al-Juzjani says he treated himself too aggressively, grew weak, and in the end gave up treatment. He died at Hamadan in June 1037 and was buried there. The present mausoleum in Hamadan was built in the twentieth century.

Avicenna in two worlds

In the Islamic world

Ibn Sina’s system replaced Aristotle as the starting point of philosophy in Arabic and Persian. His pupils included al-Juzjani, Bahmanyar and Ibn Zayla. Al-Ghazali attacked his metaphysics in The Incoherence of the Philosophers (1095); Fakhr al-Din al-Razi wrote a critical commentary on the Isharat, and Nasir al-Din al-Tusi answered it in a commentary of his own. Ibn Rushd (Averroes) in Cordoba criticized him for straying from Aristotle. Omar Khayyam called him his master and translated one of his sermons into Persian. Abu’l-Barakat al-Baghdadi reworked his theory of mayl. In logic, his propositions and syllogisms remained the core of teaching in madrasas into modern times.

In Latin Europe

• Philosophy. At Toledo in the twelfth century, Avendauth and Dominicus Gundissalinus translated the psychology and other parts of the Shifaʾ; the physics appeared as the Sufficientia. His psychology and metaphysics shaped Albertus Magnus and Thomas Aquinas.
• Medicine. Gerard of Cremona translated the Canon in the twelfth century. It became a standard text in medical faculties from the thirteenth century; there were printed Latin editions from 1472–73 and more than a dozen in the sixteenth century, and it was still read at Padua in the early eighteenth century.
• Physics. The Latin Sufficientia carried the theory of an imparted, self-preserving inclination to readers in the Latin West, where the Paris masters of the fourteenth century developed impetus theory.

Dante placed Avicenna in Limbo, between Hippocrates and Galen. A lunar crater bears his name, and UNESCO’s Avicenna Prize for Ethics in Science has been awarded since 2003.

Connections

People linked to Ibn Sina across this collection. A name without a link has no page here yet. See Ibn Sina in the network of all 337 lives →

Teachers and advisors

• Abu 'Abd Allah al-Natili

Students

• Abu 'Ubayd al-Juzjani 1012-1037
• Bahmanyar ibn Marzuban
• Ibn Zayla

Influenced by

• Aristotle
• Claudius Ptolemy
• Euclid
• John Philoponus
• al-Farabi
• Galen

Influenced

• Jean Buridan
• Nasir al-Din al-Tusi
• Omar Khayyam
• Abu'l-Barakat al-Baghdadi
• al-Ghazali
• Fakhr al-Din al-Razi
• Ibn Rushd (Averroes)
• Thomas Aquinas

Colleagues and collaborators

• Ibn al-Haytham

Rivals

• al-Biruni

Also mentioned in this life

• Archimedes
• René Descartes
• Johannes Kepler
• Isaac Newton

Fields

• natural philosophy 12 lives
• medicine 8 lives
• logic 8 lives
• optics 38 lives
• mechanics 27 lives
• astronomy 50 lives

On CoursesHub

• Optics & Photonics
• Classical Mechanics
• Astrophysics for Beginners

Sources and further reading

Biography

1. W. E. Gohlman, The Life of Ibn Sina: A Critical Edition and Annotated Translation, SUNY Press, 1974 (the autobiography and al-Juzjani’s continuation).
2. D. Gutas, Ibn Sina [Avicenna], Stanford Encyclopedia of Philosophy.
3. D. Gutas, Avicenna and the Aristotelian Tradition, 2nd ed., Brill, 2014.
4. J. J. O’Connor and E. F. Robertson, Abu Ali al-Husain ibn Abdallah ibn Sina (Avicenna), MacTutor.
5. Avicenna, Wikipedia.
6. J. McGinnis, Avicenna, Oxford University Press, 2010.

Natural philosophy, logic and medicine

1. Avicenna, The Physics of The Healing, ed. and trans. J. McGinnis, 2 vols., Brigham Young University Press, 2009.
2. J. McGinnis, Arabic and Islamic Natural Philosophy and Natural Science and related entries, Stanford Encyclopedia of Philosophy.
3. N. M. Şahin, Violent motion and mayl in Ibn Sina, Nazariyat 12, no. 1.
4. A. Sayılı, Ibn Sina and Buridan on the motion of the projectile, Annals of the New York Academy of Sciences 500 (1987).
5. Theory of impetus, Wikipedia.
6. D. C. Lindberg, Theories of Vision from al-Kindi to Kepler, University of Chicago Press, 1976.
7. R. Strobino, Ibn Sina’s Logic, Stanford Encyclopedia of Philosophy.
8. Avicennian logic (overview), Wikipedia.
9. The Canon of Medicine, Wikipedia.
10. N. G. Siraisi, Avicenna in Renaissance Italy: The Canon and Medical Teaching in Italian Universities after 1500, Princeton University Press, 1987.