Muhammad ibn Zakariyā Rāzī (Mohammad-e Zakariā-ye Rāzi: Persian: محمد زکریای رازی), known as Rhazes or Rasis after medieval Latinists, (August 28, 865, Rey – 925, Rey) was a Persian physician, alchemist and chemist, philosopher, and scholar. He is recognised as a polymath, and Biographies of Razi, based on his writings, describe him as “perhaps the greatest clinician of all times.” Numerous “firsts” in medical research, clinical care, and chemistry are attributed to him, including being the first to differentiate smallpox from measles, and the discovery of numerous compounds and chemicals including alcohol and kerosene, among others. Edward Granville Browne considers him as "probably the greatest and most original of all the physicians, and one of the most prolific as an author".

Razi made fundamental and enduring contributions to the fields of medicine, alchemy, music, and philosophy, recorded in over 200 books and articles in various fields of science. He was well-versed in Persian, Greek and Indian medical knowledge and made numerous advances in medicine through own observations and discoveries. Educated in music, mathematics, philosophy, and metaphysics, he chose medicine as his professional field. As a physician, he was an early proponent of experimental medicine and has been described as the father of pediatrics. He was also a pioneer of neurosurgery and ophthalmology. He was among the first to use Humoralism to distinguish one contagious disease from another. In particular, Razi was the first physician to distinguish smallpox and measles through his clinical characterization of the two diseases. As an alchemist, Razi is known for his study of sulfuric acid and for his discovery of ethanol and its refinement to use in medicine. He became chief physician of Rey and Baghdad hospitals. Razi invented what today is known as rubbing alcohol. Razi was a rationalist and very confident in the power of ratiocination; he was widely regarded by his contemporaries and biographers as liberal, free of prejudice, and bold in expressing his ideas. He traveled extensively, mostly in Persia. As a teacher in medicine, he attracted students of all disciplines and was said to be compassionate and devoted to the service of his patients, whether rich or poor.

Rhazes was born on 28 August 865 CE and died on 6 October 925 CE. His name Razi in Persian means from the city of Rey, an ancient town called Ragha in old Persian and Ragâ in Avestan. It is located on the southern slopes of the Elburz Range situated near Tehran, Iran. In this city (like Ibn Sina) he accomplished most of his work.

In his early life he could have been a musician or singer (see Ibn abi Usaibi'ah) but more likely a lute-player who shifted his interest from music to alchemy (cf. ibn Juljul, Sa'id, ibn Khallikan, Usaibi'ah, al-Safadi). At the age of 30 (Safadi says after 40) he stopped his study of alchemy because his experimentation had caused an eye-disease (Cf. al-Biruni), obliging him to search for physicians and medicine to cure it. al-Biruni, Beyhaqi and others, say this was the reason why he began his medical studies.

He studied medicine under Ali ibn Sahl Rabban al-Tabari, known as Ali ibn Rabban al-Tabari or Ali ibn Sahl, (Cf. al-Qifti, Usaibi'ah), a physician and philosopher born in Merv about 192 AH (808 C.E.) (d. approx. 240 AH (855 C.E.)). Ali ibn Sahl belonged to the medical school of Tabaristan or Hyrcania).

Razi became famous in his native city as a physician. He became Director of the hospital of Rey (Cf. ibn Juljul, al-Qifti, ibn abi Usaibi'ah), during the reign of Mansur ibn Ishaq ibn Ahmad ibn Asad who was Governor of Rey from 290 - 296 AH (902 - 908 C.E.) on behalf of his cousin Ahmad ibn Isma'il ibn Ahmad, second Samanian ruler. Razi dedicated his al-Tibb al-'Mansuri to Mansur ibn Ishaq ibn Ahmad, which was verified in a handwritten manuscript of his book. This was refuted by ibn al-Nadim', but al-Qifti and ibn abi Usaibi'ah confirmed that the named Mansur was indeed Mansur ibn Isma'il who died in 365 AH (975 C.E.). Razi moved from Rey to Baghdad during Caliph Muktafi's reign (approx. 289 - 295 AH (901 - 907 C.E.)) where he again held a position as Chief Director of a hospital.

After al-Muktafi's death in 295 AH (907 C.E.) Razi allegedly returned to Rey where he gathered many students around him. As Ibn al-Nadim relates in Fihrist, Razi was then a Shaikh (title given to one entitled to teach), surrounded by several circles of students. When someone arrived with a scientific question, this question was passed on to students of the 'first circle'. If they did not know the answer, it was passed on to those of the 'second circle'... and so on and on, until at last, when all others had failed to supply an answer, it came to Razi himself. We know of at least one of these students who became a physician. Razi was a very generous man, with a humane behavior towards his patients, and acting charitable to the poor. He used to give them full treatment without charging any fee, nor demanding any other payment.

His eye affliction started with cataracts and ended in total blindness. One of his pupils from Tabaristan came to look after him, but, according to al-Biruni, he refused to be treated, proclaiming it was useless as his hour of death was approaching. Some days later he died in Rey, on the 5th of Sha'ban 313 AH (27 October, 925 C.E.). However, his fame spread and lived on. In an undated catalogue of the library at Peterborough Abbey, most likely from the 14th century, he is listed as a part author of ten books on medicine.

Razi studied medicine under Ali ibn Rabban al-Tabari, however, Ibn al-Nadim indicates that he studied philosophy under al-Bakhi, who had travelled much and possessed great knowledge of philosophy and ancient sciences. Some even say that Razi attributed some of al-Balkhi's books on philosophy to himself. We know nothing about this man called al-Balkhi, not even his full name.

Razi's opponents, on the contrary, are well-known. They are the following:

• Abu al-Rabban al-Balki, chief of the Mu'tazilah of Baghdad (d. 319 AH/931 CE), a contemporary of Razi who wrote many refutations about Razi's books, especially in his Ilm al-Ilahi. His disagreements with Razi entailed his thoughts on the concept of 'Time'.
• Shuhaid ibn al-Husain al-Balkhi, with whom Razi had many controversies; one of these was on the concept of 'Pleasure', expounded in his Tafdll Ladhdhat al-Nafs which abu Sulaiman al-Mantiqi al-Sijistani quotes in his work Siwan al-Hikmah. Al-Balkhi died prior to 329 AH/940 CE.
• Abu Hatim al-Razi (Ahmad ibn Hamdan), an Isma'ili missionary, was one of his most influential opponents (d. 322 AH/933 - 934 CE). He published his controversies with Razi in his book A'lam al-Nubuwwah. Because of this book, Razi's thoughts on Prophets and Religion are preserved to the present time.
• Ibn al-Tammar (seemingly being abu Bakr Husain al-Tammar, according to Kraus) was a physician who had some disputes with Razi, as documented by Abu Hatim al-Razi in A'lam al-Nubuwwah. Ibn al-Tammar disagreed with Razi's book al-Tibb al-Ruhani but Razi rebutted him in two antitheses: First refutation of al-Tammar's disagreement with Misma'i concerning 'Matter'. Second refutation of al-Tammar's opinion of 'the Atmosphere of subterranean habitations'.

• Following are authors as described by Razi in his writings:
  • Al-Misma'i, a Mutakallim, who opposed 'materialists', counteracted Razi's treatise.
  • Jarir, a physician who had a theory about 'The eating of black mulberries after consuming water-melon'.
  • Al-Hasan ibn Mubarik al-Ummi, to whom Razi wrote two epistles with commentaries.
  • Al-Kayyal, a Mutakallim: al-Razi wrote a book on about his Theory of the Imam.
  • Mansur ibn Talhah, being the author of the book "Being", which was criticized by al-Razi.
  • Muhammad ibn al-Laith al-Rasa'ili whose opposition against alchemists was disputed by al-Razi.
• Ahmad ibn al-Tayyib al-Sarakhasi (d. 286 AH/899 CE), was an older contemporary of al-Razi. Al-Razi disagreed with him on the question of 'bitter taste'. He moreover opposed his teacher Ya'qub ibn Ishaq al-Kindi, regarding his writings, in which he discredited alchemists.

More names could be added to this list of all people opposed by al-Razi, specifically the Mu'tazilah and different Mutakallimin.