Nasir al-Din al-Tusi

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Nasīr al-Dīn Tūsī
Nasir al-Din Tusi.jpg
Iranian stamp for the 700th anniversary of his death
TitleKhawaja Nasir
Born18 February 1201
Tus, Khorasan
Died26 June 1274(1274-06-26) (aged 73)
Al-Kadhimiya Mosque, Kadhimiya, Baghdad, Ilkhanate
EraIslamic Golden Age
Main interest(s)Kalam, Islamic Philosophy, Astronomy, Mathematics, Biology and Medicine, Physics, Science
Notable idea(s)Spherical trigonometry, Tusi couple
Notable work(s)Rawḍa-yi Taslīm, Tajrīd al-'Aqa'id,
Akhlaq-i-Nasri, Zij-i ilkhani,
al-Risalah al-Asturlabiyah,
Al-Tadhkirah fi'ilm al-hay'ah
TeachersKamal al-Din Yunus[1]
Senior posting

Muhammad ibn Muhammad ibn al-Hasan al-Tūsī (Persian: محمد بن محمد بن حسن طوسی‎‎ 24 February 1201 – 26 June 1274), better known as Nasir al-Din Tusi (Persian: نصیر الدین طوسی‎; or simply Tusi /ˈtsi/[3] in the West), was a Persian polymath, architect, philosopher, physician, scientist, and theologian.[4][5][6][7][8][9][10][11][12][13] He established trigonometry as an independent branch of mathematics.[14][15][16] He was a Twelver Shia Muslim.[17] Ibn Khaldun (1332–1406) claimed Tusi was the greatest of the later Persian scholars.[18]


Al-Tūsī was born in the city of Tus, Khorasan (now in northeastern Iran) in the year 1201 and began his studies at an early age. In Hamadan and Tus he studied the Quran, hadith, Ja'fari jurisprudence, logic, philosophy, mathematics, medicine and astronomy.[19]

Muhammad al-Tūsī's family was Shī‘ah. His father, who died while Muhammad was a only boy, had, according to his Islamic faith, encouraged his son to study. Al-Tūsī took the acquisition of knowledge very seriously and traveled far and wide attending the lectures of renowned scholars. At a young age, he moved to Nishapur to study philosophy under Farid al-Din Damad and mathematics under Muhammad Hasib.[20] He met also Attar of Nishapur, the legendary Sufi master who was later killed by the Mongols, and he attended the lectures of Qutb al-Din al-Misri.

In Mosul al-Tūsī studied mathematics and astronomy with Kamal al-Din Yunus (d. AH 639 / AD 1242), a pupil of Sharaf al-Dīn al-Ṭūsī.[1] Later he corresponded with Sadr al-Din al-Qunawi, the son-in-law of Ibn Arabi, and it seems that the mysticism of the Sufi masters of his time, did not appeal to him. However he composed his own manual of philosophical Sufism in the form of a small booklet entitled Awsaf al-Ashraf "The Attributes of the Illustrious".

As the armies of Genghis Khan swept across his homeland, al-Tūsī was employed by the Nizari Ismaili state. His most important contributions to science were made while he moved from one stronghold to another.[21] He was captured after the invasion of Alamut Castle by the Mongol forces.[22]


Tusi couple from Vat. Arabic ms 319

While in Nishapur, al-Tūsī established a reputation for scholarship. He composed over 150 prose works, and his diwan (collected works) is one of the largest by any Muslim author. Writing in Arabic and Persian, Nasir al-Din Tusi treats religious ("Islamic") and secular subjects ("the ancient sciences").[23] He translated the works of Euclid, Archimedes, Ptolemy, Autolycus, and Theodosius of Bithynia into Arabic.[23]


The Astronomical Observatory of Nasir al-Dīn Tusi.

When al-Tūsī convinced the Mongol conqueror Hulegu Khan to build an observatory for purpose of producing accurate astronomical tables, it was the most technically advanced in the world. Astronomical charts were used in astrological predictions and to calculate the dates of religious festivals, in addition to many other civil, military and navigational applications. Begun in 1259, the Rasad Khaneh observatory was constructed in Azarbaijan, south of the river Aras and west of Maragheh, the capital of the Ilkhanate Empire.[24]

Using the sophisticated measurements taken at the observatory Al-Tusi's highly accurate astronomical tables of planetary movements are depicted in his book Zij-i ilkhani (Ilkhanic Tables). His tables calculating the positions of the planets and the names of the stars and his model for the planetary system, the most scientifically advanced of its time, were in use until the development of the heliocentric model in the time of Nicolaus Copernicus. Between Ptolemy and Copernicus, he is considered one of the most eminent astronomers. His famous student Shams ad-Din Al-Bukhari [2] taught the Byzantine scholar Gregory Chioniadis,[25] who had in turn trained the astronomer Manuel Bryennios [26] about 1300 in Constantinople.


The Tusi-couple is a geometrical technique invented by al-Ṭūsī to generate linear motion from the sum of two circular motions and is used in his planetary models. The technique replaced Ptolemy's problematic equant[27] for many planets, but was unable to find a solution to Mercury, which was solved later by Ibn al-Shatir as well as Ali Qushji.[28] The Tusi couple was later employed in Ibn al-Shatir's geocentric model and Nicolaus Copernicus' heliocentric Copernican model.[29] He also calculated the value for the annual precession of the equinoxes and contributed to the construction and usage of some astronomical instruments including the astrolabe.

Al-Ṭūsī noted that Ptolemy's observations for the Earth's being at rest, were not evidence of proof. However while he too believed in a fixed earth, as did his 16th-century commentator al-Bīrjandī, he insisted only the physical principles based on natural philosophy could be relied upon.[30] Tusi's criticisms of Ptolemy were similar to the arguments later used by Copernicus in 1543 to defend the Earth's rotation.[31]

On the real essence of the Milky Way, Ṭūsī in his Tadhkira writes: "The Milky Way, i.e. the galaxy, is made up of a very large number of small, tightly-clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. because of this, it was likened to milk in color." [32] Three centuries later the proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it is really composed of a huge number of faint stars.[33]


Al-Tūsī was a supporter of Avicennian logic, and wrote the following commentary on Avicenna's theory of absolute propositions:

"What spurred him to this was that in the assertoric syllogistic Aristotle and others sometimes used contradictories of absolute propositions on the assumption that they are absolute; and that was why so many decided that absolutes did contradict absolutes. When Avicenna had shown this to be wrong, he wanted to develop a method of construing those examples from Aristotle."[34]


A stamp issued in the republic of Azerbaijan in 2009 honoring Tusi

Al-Tusi's extensive exposition Treatise on the Quadrilateral, distinguish spherical trigonometry from astronomy.[35][36] It was in the works of Al-Tusi that trigonometry achieved the status of an independent branch of pure mathematics distinct from astronomy, to which it had been linked for so long.[37][38]

He created a list of six cases for the right triangle in spherical trigonometry.[39]

His discoveries in spherical trigonometry followed the tradition of earlier Greek mathematicians such as Menelaus of Alexandria, who wrote Sphaerica, and the Muslim mathematicians Abū al-Wafā' al-Būzjānī and Al-Jayyani.

In his On the Sector Figure, appears the famous law of sines for plane triangles.[40]

He also stated the law of sines for spherical triangles,[41][42] discovered the law of tangents for spherical triangles, and provided proofs for these laws.[40]


In his Akhlaq-i Nasiri, Tusi wrote about several biological topics. He defended a version of Aristotle's scala naturae, in which he placed man above animals, plants, minerals, and the elements. He described "grasses which grow without sowing or cultivation, by the mere mingling of elements,"[43] as closest to minerals. Among plants, he considered the date-palm as the most highly developed, since "it only lacks one thing further to reach (the stage of) an animal: to tear itself loose from the soil and to move away in the quest for nourishment."[43]

The lowest animals "are adjacent to the region of plants: such are those animals which propagate like grass, being incapable of mating [...], e.g. earthworms, and certain insects".[44] The animals "which reach the stage of perfection [...] are distinguished by fully developed weapons", such as antlers, horns, teeth, and claws. Tusi described these organs as adaptations to each species's lifestyle, in a way anticipating natural theology. He continued:

"The noblest of the species is that one whose sagacity and perception is such that it accepts discipline and instruction: thus there accrues to it the perfection not originally created in it. Such are the schooled horse and the trained falcon. The greater this faculty grows in it, the more surpassing its rank, until a point is reached where the (mere) observation of action suffices as instruction: thus, when they see a thing, they perform the like of it by mimicry, without training [...]. This is the utmost of the animal degrees, and the first of the degrees of Man in contiguous therewith."[45]

In this paragraph, al-Tusi classifies types of learning, recognising observational learning as the most advanced form, and correctly attributing it to certain animals. In his statement: "the Animal Soul [comprising the faculties of perception and movement ...] is restricted to individuals of the animal species", and that, by possessing a "Human Soul, [...] mankind is distinguished and particularized among other animals." al-Tusi classes man among the animals, [46]

Al-Tusi's biological writings have been interpreted as a kind of proto-evolutionary theory.[47][48] However, Tusi did not state explicitly that he believed species to change over time.


Al-Tusi's main contribution to chemistry, is his formulation of an early law of conservation of mass.[49]


A 60-km diameter lunar crater located on the southern hemisphere of the moon is named after him as "Nasireddin". A minor planet 10269 Tusi discovered by Soviet astronomer Nikolai Stepanovich Chernykh in 1979 is named after him.[50][51] The K. N. Toosi University of Technology in Iran and Observatory of Shamakhy in the Republic of Azerbaijan are also named after him. In February 2013, Google celebrated his 812th birthday with a doodle, which was accessible in its websites with Arabic language calling him al-farsi (the Persian).[52][53]


Of about 150 works al-Tusi wrote, 25 were in Persian, the remaining in Arabic,[23] and one treatise in Persian, Arabic and Turkish.[54]

A Treatise on the Astrolabe by Tusi, Isfahan 1505

Selected major works:

  • Kitāb al-Shakl al-qattāʴ (كتاب شكل القطاع) 'Book of the Complete Quadrilateral'; A five-volume summary of trigonometry. Traité Du Quadrilatère French-Arabic edition (Constantinople, 1891)[55]
  • Al-Tadhkirah fi'ilm al-hay'ah – A memoir on the science of astronomy. Many commentaries were written about this work called Sharh al-Tadhkirah (A Commentary on al-Tadhkirah) - Commentaries were written by Abd al-Ali ibn Muhammad ibn al-Husayn al-Birjandi and by Nazzam Nishapuri.
  • Akhlaq-i Nasiri – A work on ethics.
  • al-Risalah al-Asturlabiyah – A Treatise on the astrolabe.
  • Zij-i Ilkhani (Ilkhanic Tables) – A major astronomical treatise, completed in 1272.
  • Sharh al-Isharat (Commentary on Avicenna's Isharat)
  • Awsaf al-Ashraf a short mystical-ethical work in Persian
  • Tajrīd al-Iʿtiqād (Summation of Belief) – A commentary on Shia doctrines.
  • Talkhis al-Muhassal (summary of summaries).
  • Tadhkira
  • Maʾkhūdhāt mansūba ilā Arshimīdis (مأخوذات منسوبة إلى أَ ْرشَمِيْدس); a Latin-English translation by John Greaves titled Lemmata Archimidis e vetusto codice manuscripto Arabico (London, 1659) introduced the work into Europe.[56]


An example from one of his poems:

Anyone who knows, and knows that he knows,
makes the steed of intelligence leap over the vault of heaven.
Anyone who does not know but knows that he does not know,
can bring his lame little donkey to the destination nonetheless.
Anyone who does not know, and does not know that he does not know,
is stuck forever in double ignorance.

See also[edit]


  1. ^ a b Sharaf al-Din al-Muzaffar al-Tusi biography - MacTutor History of Mathematics
  2. ^ a b Nasir al-Din al-Tusi at the Mathematics Genealogy Project
  3. ^ "Tusi". Random House Webster's Unabridged Dictionary.
  4. ^ Bennison, Amira K. (2009). The great caliphs : the golden age of the 'Abbasid Empire. New Haven: Yale University Press. p. 204. ISBN 978-0-300-15227-2. Hulegu killed the last ‘Abbasid caliph but also patronized the foundation of a new observatory at Maragha in Azerbayjan at the instigation of the Persian Shi‘i polymath Nasir al-Din Tusi.
  5. ^ Goldschmidt, Arthur; Boum, Aomar (2015). A Concise History of the Middle East. Avalon Publishing. ISBN 978-0-8133-4963-3. Hulegu, contrite at the damage he had wrought, patronized the great Persian scholar, Nasiruddin Tusi (died 1274), who saved the lives of many other scientists and artists, accumulated a library of 400000 volumes, and built an astronomical ...
  6. ^ Bar Hebraeus; Joosse, Nanne Pieter George (2004). A Syriac Encyclopaedia of Aristotelian Philosophy: Barhebraeus (13th C.), Butyrum Sapientiae, Books of Ethics, Economy, and Politics : a Critical Edition, with Introduction, Translation, Commentary, and Glossaries. Brill. p. 11. ISBN 978-90-04-14133-9. the Persian scholar Naṣīr al-Dīn al-Ṭūsī
  7. ^ Seyyed Hossein Nasr (2006). Islamic Philosophy from Its Origin to the Present: Philosophy in the Land of Prophecy. State University of New York Press. p. 167. ISBN 978-0-7914-6800-5. In fact it was common among Persian Islamic philosophers to write few quatrains on the side often in the spirit of some of the poems of Khayyam singing about the impermanence of the world and its transience and similar themes. One needs to only recall the names of Ibn Sina, Suhrawardi, Nasir al-Din Tusi and Mulla Sadra, who wrote poems along with extensive prose works.
  8. ^ Rodney Collomb, "The rise and fall of the Arab Empire and the founding of Western pre-eminence", Published by Spellmount, 2006. pg 127: "Khawaja Nasr ed-Din Tusi, the Persian, Khorasani, former chief scholar and scientist of"
  9. ^ Seyyed Hossein Nasr, Islamic Philosophy from Its Origin to the Present: Philosophy in the Land of Prophecy, SUNY Press, 2006, ISBN 0-7914-6799-6. page 199
  10. ^ Seyyed H. Badakhchani. Contemplation and Action: The Spiritual Autobiography of a Muslim Scholar: Nasir al-Din Tusi (In Association With the Institute of Ismaili Studies. I. B. Tauris (December 3, 1999). ISBN 1-86064-523-2. page.1: ""Nasir al-Din Abu Ja`far Muhammad b. Muhammad b. Hasan Tusi:, the renowned Persian astronomer, philosopher and theologian"
  11. ^ Glick, Thomas F.; Livesey, Steven John; Wallis, Faith (2005). Medieval Science, Technology, and Medicine: An Encyclopedia. Psychology Press. ISBN 978-0-415-96930-7. drawn by the Persian cosmographer al-Tusi.
  12. ^ Laet, Sigfried J. de (1994). History of Humanity: From the seventh to the sixteenth century. UNESCO. p. 908. ISBN 978-92-3-102813-7. the Persian astronomer and philosopher Nasir al-Din Tusi.
  13. ^ Mirchandani, Vinnie (2010). The New Polymath: Profiles in Compound-Technology Innovations. John Wiley & Sons. p. 300. ISBN 978-0-470-76845-7. Nasir. al-Din. al-Tusi: Stay. Humble. Nasir al-Din al-Tusi, the Persian polymath, talked about humility: “Anyone who does not know and does not know that he does not know is stuck forever in double ...
  14. ^ "Al-Tusi_Nasir biography". Retrieved 2018-08-05. One of al-Tusi's most important mathematical contributions was the creation of trigonometry as a mathematical discipline in its own right rather than as just a tool for astronomical applications. In Treatise on the quadrilateral al-Tusi gave the first extant exposition of the whole system of plane and spherical trigonometry. This work is really the first in history on trigonometry as an independent branch of pure mathematics and the first in which all six cases for a right-angled spherical triangle are set forth.
  15. ^ "the cambridge history of science".
  16. ^ "ṬUSI, NAṢIR-AL-DIN i. Biography – Encyclopaedia Iranica". Retrieved 2018-08-05. His greatest contribution to mathematics (Nasr, 1996, pp. 208-14) was establishing a new discipline of trigonometry. He developed spherical trigonometry and the six fundamental formulas for the solution of spherical right-angled triangles.
  17. ^ Ṭūsī, Naṣīr al-Dīn Muḥammad ibn Muḥammad; Badakchani, S. J. (2005), Paradise of Submission: A Medieval Treatise on Ismaili Thought, Ismaili Texts and Translations, 5, London: I.B. Tauris in association with Institute of Ismaili Studies, pp. 2–3, ISBN 1-86064-436-8
  18. ^ James Winston Morris, "An Arab Machiavelli? Rhetoric, Philosophy and Politics in Ibn Khaldun’s Critique of Sufism", Harvard Middle Eastern and Islamic Review 8 (2009), pp 242–291. [1] excerpt from page 286 (footnote 39): "Ibn Khaldun’s own personal opinion is no doubt summarized in his pointed remark (Q 3: 274) that Tusi was better than any other later Iranian scholar". Original Arabic: Muqaddimat Ibn Khaldūn : dirāsah usūlīyah tārīkhīyah / li-Aḥmad Ṣubḥī Manṣūr-al-Qāhirah : Markaz Ibn Khaldūn : Dār al-Amīn, 1998. ISBN 977-19-6070-9. Excerpt from Ibn Khaldun is found in the section: الفصل الثالث و الأربعون: في أن حملة العلم في الإسلام أكثرهم العجم (On how the majority who carried knowledge forward in Islam were Persians) In this section, see the sentence where he mentions Tusi as more knowledgeable than other later Persian ('Ajam) scholars: . و أما غيره من العجم فلم نر لهم من بعد الإمام ابن الخطيب و نصير الدين الطوسي كلاما يعول على نهايته في الإصابة. فاعتير ذلك و تأمله تر عجبا في أحوال الخليقة. و الله يخلق ما بشاء لا شريك له الملك و له الحمد و هو على كل شيء قدير و حسبنا الله و نعم الوكيل و الحمد لله.
  19. ^ Dabashi, Hamid. "Khwajah Nasir al-Din Tusi: The philosopher/vizier and the intellectual climate of his times". Routledge History of World Philosophies. Vol I. History of Islamic Philosophy. Seyyed Hossein Nasr and Oliver Leaman (eds.) London: Routledge. 1996. p. 529
  20. ^ Siddiqi, Bakhtyar Husain. "Nasir al-Din Tusi". A History of Islamic Philosophy. Vol 1. M. M. Sharif (ed.). Wiesbaden:: Otto Harrossowitz. 1963. p. 565
  21. ^ Peter Willey, The Eagle's Nest: Ismaili Castles in Iran and Syria, (I.B. Tauris, 2005), 172.
  22. ^ Michael Axworthy, A History of Iran: Empire of the Mind, (Basic Books, 2008), 104.
  23. ^ a b c H. Daiber, F.J. Ragep, "Tusi" in Encyclopaedia of Islam. Edited by: P. Bearman, Th. Bianquis, C.E. Bosworth, E. van Donzel and W.P. Heinrichs. Brill, 2007. Brill Online. Quote: "Tusi's prose writings, which number over 150 works, represent one of the largest collections by a single Islamic author. Writing in both Arabic and Persian, Nasir al-Din dealt with both religious ("Islamic") topics and non-religious or secular subjects ("the ancient sciences")."
  24. ^ Morris Rossabi (28 November 2014). From Yuan to Modern China and Mongolia: The Writings of Morris Rossabi. BRILL. pp. 281–. ISBN 978-90-04-28529-3.
  25. ^ Nasir al-Din al-Tusi at the Mathematics Genealogy Project
  26. ^ Nasir al-Din al-Tusi at the Mathematics Genealogy Project
  27. ^ Craig G. Fraser, 'The cosmos: a historical perspective', Greenwood Publishing Group, 2006 p.39
  28. ^ George Saliba, 'Al-Qushji's Reform of the Ptolemaic Model for Mercury', Arabic Sciences and Philosophy, v.3 1993, pp.161-203
  29. ^ George Saliba, 'Revisiting the Astronomical Contacts Between the World of Islam and Renaissance Europe: The Byzantine Connection', 'The occult sciences in Byzantium', 2006, p.368
  30. ^ Ragep, F. Jamil (2001), "Freeing Astronomy from Philosophy: An Aspect of Islamic Influence on Science", Osiris, 16, 2nd ser.: 49–64, Bibcode:2001Osir...16...49R, doi:10.1086/649338, JSTOR 301979, at p. 60.
  31. ^ F. Jamil Ragep (2001), "Tusi and Copernicus: The Earth's Motion in Context", Science in Context 14 (1-2), p. 145–163. Cambridge University Press.
  32. ^ Ragep, Jamil, Nasir al-Din Tusi’s Memoir on Astronomy (al-Tadhkira fi `ilm al-hay’ a) Edition, Translation, Commentary, and Introduction. 2 vols. Sources in the History of Mathematics and Physical Sciences. New York: Springer-Verlag, 1993. pp. 129
  33. ^ O'Connor, J. J.; Robertson, E. F. (November 2002). "Galileo Galilei". University of St Andrews. Retrieved 2007-01-08.
  34. ^ Tony Street (July 23, 2008). "Arabic and Islamic Philosophy of Language and Logic". Stanford Encyclopedia of Philosophy. Retrieved 2008-12-05.
  35. ^ "trigonometry". Encyclopædia Britannica. Retrieved 2011-04-25.
  36. ^ * Katz, Victor J. (1993). A History of Mathematics: An Introduction, p259. Addison Wesley. ISBN 0-673-38039-4.
  37. ^ Bosworth, Clifford E.; Asimov (2003). History of civilizations of Central Asia. 4. Motilal Banarsidass. p. 190. ISBN 81-208-1596-3.
  38. ^ Hayes, John R.; Badeau, John S. (1983). The genius of Arab civilization : source of Renaissance (2nd ed.). Taylor & Francis. p. 156. ISBN 0-262-08136-9.
  39. ^,"One of al-Tusi's most important mathematical contributions was the creation of trigonometry as a mathematical discipline in its own right rather than as just a tool for astronomical applications. In Treatise on the quadrilateral al-Tusi gave the first extant exposition of the whole system of plane and spherical trigonometry. This work is really the first in history on trigonometry as an independent branch of pure mathematics and the first in which all six cases for a right-angled spherical triangle are set forth"/
  40. ^ a b Berggren, J. Lennart (2007). "Mathematics in Medieval Islam". The Mathematics of Egypt, Mesopotamia, China, India, and Islam: A Sourcebook. Princeton University Press. p. 518. ISBN 978-0-691-11485-9.
  41. ^ Also the 'sine law' (of geometry and trigonometry, applicable to spherical trigonometry) is attributed, among others, to al-Khujandi. (The three others are Abul Wafa Bozjani, Nasiruddin Tusi, and Abu Nasr Mansur). Razvi, Syed Abbas Hasan (1991) A history of science, technology, and culture in Central Asia, Volume 1 University of Peshawar, Peshawar, Pakistan, p.358, OCLC 26317600
  42. ^ Bijli suggests that three mathematicians are in contention for the honor, Alkhujandi, Abdul-Wafa and Mansur, leaving out Nasiruddin Tusi. Bijli, Shah Muhammad and Delli, Idarah-i Adabiyāt-i (2004) Early Muslims and their contribution to science: ninth to fourteenth century Idarah-i Adabiyat-i Delli, Delhi, India, p.44, OCLC 66527483
  43. ^ a b Nasir ad-Din Tusi (1964) The Nasirean Ethics (translator: G.M. Wickens). London: Allen & Unwin, p. 44.
  44. ^ Nasir ad-Din Tusi (1964) The Nasirean Ethics (translator: G.M. Wickens). London: Allen & Unwin, p. 45.
  45. ^ Nasir ad-Din Tusi (1964) The Nasirean Ethics (translator: G.M. Wickens). London: Allen & Unwin, p. 45f.
  46. ^ Nasir ad-Din Tusi (1964) The Nasirean Ethics (translator: G.M. Wickens). London: Allen & Unwin, p. 42 (emphasis added).
  47. ^ Alakbarli, Farid (Summer 2001). "A 13th-Century Darwin? Tusi's Views on Evolution". Azerbaijan International. 9 (2): 48–49.
  48. ^ Shoja, M.M.; Tubbs, R.S. (2007). "The history of anatomy in Persia". Journal of Anatomy. 210: 359–378. doi:10.1111/j.1469-7580.2007.00711.x. PMC 2100290.
  49. ^ Alakbarli, Farid (2001). "A 13th-Century Darwin? Tusi's Views on Evolution". Azerbaijan International. 9 (2 (Summer 2001)): 48–49. Retrieved 27 January 2018. While this reasoning may seem backward to today's Western mind, some of Tusi's theories did have merit. For instance, Tusi believed that a body of matter is able to change, but is not able to entirely disappear. He wrote: 'A body of matter cannot disappear completely. It only changes its form, condition, composition, color and other properties and turns into a different complex or elementary matter'.
  50. ^ "2003ASPC..289..157B Page 157". Retrieved 2013-02-27.
  51. ^ 10269 tusi - Mano biblioteka - Google knygos. Retrieved 2013-02-27.
  52. ^ "Nasir al-Din al-Tusi's 812th Birthday". Google. Retrieved 19 February 2013.
  53. ^ "In Persian نگاه عربی به خواجه نصیرالدین طوسی در گوگل". 19 February 2013. Retrieved 19 February 2013.
  54. ^ Seyyed Hossein Nasr. The Islamic Intellectual Tradition in Persia. Curson Press, 1996. See p. 208: "Nearly 150 treatises and letters by Nasir al-Din Tusi are known, of which 25 are in Persian and the rest in Arabic. There is even a treatise on geomancy which Tusi wrote in Arabic, Persian, and Turkish, demonstrating his mastery of all three languages."
  55. ^ Traité Du Quadrilatère Attribué A Nassiruddinel-toussy, D'après Un Manuscrit Tiré De La Bibliothèque De S.a. Edhem Pacha by Naṣīr al-Dīn Muḥammad ibn Muḥamm, Alexander Karathéodory (pasha.) (ISBN: 9781175947314) internet archive
  56. ^ The Correspondence of James Ussher 1600-1656, vol.III, p.879, n.2,3

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