Philosophical and Theological Thought in Shi'ism
It has been mentioned before that Islam has legitimized and approved rational thought, which it considers a part of religious thought. Rational thought in its Islamic sense, after confirming the prophecy of the Prophet, provides intellectual demonstrations of the validity of the external aspect of the Quran, which is a divine revelation, as well as of the definitely established sayings of the Prophet and his noble Household.
Intellectual proofs, which aid man in finding solutions for these problems through his God-given nature, are of two kinds : demonstration (burhan) and dialectic (jadal). Demonstration is a proof whose premises are true (accord with reality) even if they be not observable or evident.
In other words, it is a proposition which man comprehends and confirms by necessity through his God-given intelligence, as for example when he knows that "the number three is less than four."
This type of thought is called rational thought; and in case it concerns universal problems of existence, such as the origin and end of the world and of man, it becomes known as philosophical thought.
Dialectic is a proof all or some of whose premises are based on observable and certain data, as for example the case of believers in a religion for whom the common practice is to prove their religious views within that religion by appealing to its certain and evident principles.
The Holy Quran has employed both these methods and there are many verses in the Holy Book attesting to each type of proof.
First of all, the Quran commands free investigation and meditation upon the universal principles of the world of existence and the general principles of cosmic order, as well as upon more particular orders such as that of the heavens, the stars, day and night, the earth, the plants, animals, men, etc. It praises in the most eloquent language intellectual investigation of these matters.
Secondly, the Quran has commanded man to apply dialectical thought, which is usually called theological (kalami) discussion, provided it is accomplished in the best manner possible, that is, with the aim of manifesting the truth without contention and by men who possess the necessary moral virtues. It is said in the Quran, "Call unto the way of thy Lord with wisdom and fair exhortation, and reason ["jadil," from jadal] with them in the better way" (Quran, XVI, 125).

Shi'ite Initiative in Islamic Philosophy and Kalam
As for theology, kalam, it is clear that from the beginning when the Shi'ites separated from the Sunni majority they began to debate with their opponents concerning their own particular point of view. It is true that a debate has two sides and that both the opponents share in it. However, the Shi'ites were continuously on the offensive, taking the initiative, while the other side played the defensive role. In the gradual growth of kalam, which reached its height in 2nd/8th and 3rd/9th centuries with the spread of the Mu'tazilite school,
Shi'ite scholars and learned men, who were students of the school of the Household of the Prophet, became among the foremost masters of kalam. Furthermore, the chain of theologians of the Sunni world, whether it be the Ash'arites, Mu'tazilites or others, goes back to the first Imam of the Shi'ites, Ali.
As for philosophy, those who are acquainted with sayings and works of the companions of the Prophet (of which the names of 12,000 have been recorded and 120,000 are known to exist) know that there is little in them containing an appreciable discussion of philosophical questions. It is only Ali whose compelling metaphysical utterances contain the deepest philosophical thought.
The companions and the scholars who followed them, and in fact the Arabs of that day in general, were not acquainted with free intellectual discussion.
There is no example of philosophical thought in the works of the scholars of the first two centuries. Only the profound sayings of the Shi'ite Imams, particularly the first and eighth, contain an inexhaustible treasury of philosophical meditations in their Islamic context. It is they who acquainted some of their students with this form of thought.
The Arabs were not familiar with philosophical thought until they saw examples of it during the 2nd/8th century in the translation of certain philosophical works into Arabic. Later, during the 3rd/9th century, numerous philosophical writings were translated into Arabic from Greek, Syriac, and other languages and through them the method of philosophical thought became known to the general public.
Nevertheless, most jurists and theologians did not look upon philosophy and other intellectual sciences, which were newly arrived guests, with favor. At the beginning, because of the support of the governmental authorities for these sciences, their opposition did not have much effect. But conditions soon changed through strict orders many philosophical works were destroyed.
The Epistles of the Brethren of Purity, which is the work of a group of unknown authors, is a reminder of those days and attests to the unfavorable conditions of that epoch.
After this period of difficulty, philosophy was revived at the beginning of the 4th/10th century by the famous philosopher Abu Nasr al-Farabi. In the 5th/11th century, as a result of the works of the celebrated philosopher Ibn Sina (Avicenna), Peripatetic philosophy reached its full development.
In the 6th/12th century Shaykh al-Ishraq Shihab al-Din Suhrawardi systematized the philosophy of illumination (ishraq) and because of this was executed by the order of Salah al-Din Ayyubi. Thereafter, philosophy ceased to exist among the Muslim majority in the Sunni world. There was no further outstanding philosopher in that part of the Muslim world except in Andalusia at the edge of the Islamic world where at the end of the 6th/12th century Ibn Rushd (Averroes) sought to revive the study of philosophy.

Shi'ite Contributions to Philosophy and the Intellectual Sciences
In the same way that from the beginning Shi'ism played an effective role in the formation of Islamic philosophical thought, it was also a principal factor in the further development and propagation of philosophy and the Islamic sciences. Although after Ibn Rushd philosophy disappeared in the Sunni world, it continued to live in Shi'ism.
After Ibn Rushd there appeared such celebrated philosophers as Khwajah Nasir al-Din Tusi, Mir Damad, and Sadr al-Din Shirazi, who studied, developed and expounded philosophical thought one after another. In the same manner, in the other intellectual sciences, there appeared many outstanding figures such as Nasir al-Din Tusi (who was both philosopher and mathematician) and Birjandi, who was also an outstanding mathematician.
All the sciences, particularly metaphysics or theosophy (falsafah-i ilahi or hikmat-i ilahi), made major advances thanks to the indefatigable endeavor of Shi'ite scholars. This fact can be seen if one compares the works of Nasir al-Din Tusi, Shams al-Din Turkah, Mir Damad, and Sadr al-Din Shirazi with the writings of those who came before them.
It is known that the element that was instrumental in the appearance of philosophical and metaphysical thought in Shi'ism and through Shi'ism in other Islamic circles was the treasury of knowledge left behind by the Imams.
The persistence and continuity of this type of thought in Shi'ism is due to the existence of this same treasury of knowledge, which Shi'ism has continued to regard with a sense of reverence and respect.
In order to clarify this situation it is enough to compare the treasury of knowledge left by the Household of the Prophet with the philosophical works written over the course of the centuries.
In this comparison one can see clearly how each day Islamic philosophy approached this source of knowledge ever more closely, until in the 11th/17th century Islamic philosophy and this inspired treasury of wisdom converged more or less completely. They were separated only by certain differences of interpretation of some of the principles of philosophy.

Outstanding Intellectual Figures of Shi'ism
Thiqat al-islam Muhammad ibn Ya'qub Kulayni (d. 329/940) is the first person in Shi'ism to have separated the Shi'ite hadiths from the books called Principles (usul) and to have arranged and organized them according the headings of jurisprudence and articles of faith. (Each one of the Shi'ite scholars of hadith had assembled sayings he had collected from the Imams in a book called Asl, or Principles.) The book of Kulayni known as Kafi is divided into three parts: Principles, Branches, and Miscellaneous Articles, and contains 16,199 hadiths. It is the most trustworthy and celebrated work of hadith known in the Shi'ite world.
Three other works which complement the Kafi are the book of the jurist Shaykh-i Saduq Muhammad ibn Babuyah Qumi(d. 381/991), and Kitab al-tahdhib and Kitab al-istibsar, both by Shaykh Muhammad Tusi (d. 460/1068).
Abu'l-Qasim Ja'far ibn Hasan ibn Yahya Hilli (d. 676/1277), known as Muhaqqiq, was an outstanding genius in the science of jurisprudence and is considered to be the foremost Shi'ite jurist.
Among his masterpieces are Kitab-i mukhtasar-i nafi' and Kitab-i sharayi', which have been passed from hand to hand for seven hundred years among Shi'ite jurists and have always been regarded with a sense of awe and wonder.
Following Muhaqqiq, we must cite Shahid-i Awwal (the First Martyr) Shams al-Din Muhammad ibn Makki, who was killed in Damascus in 786/1384 on the accusation of being Shi'ite. Among his juridical masterpieces is his Lum'ah-i dimashqiyah which he wrote in prison in a period of seven days. Also we must cite Shaykh Ja'far Kashif al-Ghita' Najafi (d. 1327/1909) among whose outstanding juridical works is Kitab kashf al-ghita'.
Khwajah Nasir al-Din Tusi (d. 672/1274) is the first to have made kalam a thorough and complete science.
Among his masterpieces in this domain is his Tajrid al-iteqed which has preserved its authority among masters of this discipline for more than seven centuries. Numerous commentaries have been written on it by Shi'ites and Sunnis alike. Over and above his genius in the science of kalam, he was one of the outstanding figures of his day in philosophy and mathematics as witnessed by the valuable contributions he made to the intellectual sciences. Moreover, the Maraghah observatory owed its existence to him.
Sadr al-Din Shirazi (d. 1050/1640), known as Mulla Sadra and Sadr al-Muta'allihin, was the philosopher who, after centuries of philosophical development in Islam,
brought complete order and harmony into the discussion of philosophical problems for the first time. He organized and systematized them like mathematical problems and at the same time wed philosophy and gnosis, thereby bringing about several important developments. He gave to philosophy new ways to discuss and solve hundreds of problems that could not be solved through Peripatetic philosophy. He made possible the analysis and solution of a series of mystical questions which to that day had been considered as belonging to a domain above that of reason and beyond comprehension through rational thought.
He clarified and elucidated the meaning of many treasuries of wisdom, contained in the exoteric sources of religion in the profound metaphysical utterances of the Imams of the Household of the Prophet, that for centuries had been considered as insoluble riddles and usually believed to be of an allegorical or even unclear nature. In this way gnosis, philosophy and the exoteric aspect of religion were completely harmonized and began to follow a single course.
By following the methods he had developed, Mulla Sadra succeeded in proving "transubstantial motion" (harakat-i jawhariyah), and in discovering the intimate relation of time to the three spatial dimensions in a manner that is similar to the meaning given in modern physics to the "fourth dimension" and which resembles the general principles of the theory of relativity (relativity of course in the corporeal world outside the mind, not in the mind), and many other noteworthy principles. He wrote nearly fifty books and treatises. Among his greatest masterpieces is the four-volume Asfar.
It should be noted here that before Mulla Sadra certain sages like Suhrawardi, the 6th/12th century philosopher and author of Hikmat al-ishraq, and Shams al-Din Turkah, a philosopher of the 8th/14th century, had taken steps toward harmonizing gnosis, philosophy and exoteric religion, but credit for complete success in this undertaking belongs to Mulla Sadra.
Shaykh Murtada Ansari Shustari (d. 1281/1864) reorganized the science of the principles of jurisprudence upon a new foundation and formulated the practical principles of this science. For over a century his school has been followed diligently by Shi'ite scholars.