The Holy Quran is the basis of the Shi'ite thought and the source of all the Shiite teachings. It considers the apparent and outward meanings of the verses, the sayings, conducts, silence and assertions of the Messenger of God (pbuh) and those of the Infallible Imams as authoritative proofs.
Additionally, the Quran has set forth the validity of intellectual arguments and endorsed intellectual intuition and unveiling.
Such a thought can be summed up and outlined as under:
- Belief in the oneness of God, the Almighty, and that He is qualified with all the attributes of perfection and exalted from all the attributes of defects;
- Belief in rational good and rational evil and that man's sound reason perceives that God does not commit an evil act;
- Belief in the infallibility of the divine prophets and finality of the Prophet of Islam;
- Belief in the fact that the Holy Prophet's successor is appointed by God through the Prophet (pbuh) or a former infallible Imam and that the number of the successors of the Prophet (pbuh) is twelve beginning with Ali bin Abi Talib (a.s.) and ending with Imam Mahdi who is alive and awaiting God's command.
- Belief in the life after death and that man is held responsible and rewarded for his deeds.
If we are to give a detailed answer to the foregoing question, we may have to write several volumes of books but for the sake of brevity we suffice to mentioning a few points:
1. The Shi'ite's only Source for Acquiring Knowledge
The single source upon which the divinely revealed religion of Islam depends and upon which it is based, inasmuch as it is based on a revelation of celestial origin, is none other than the Holy Quran. It is the Quran which is the definitive testament of the universal and ever-living prophethood of the Prophet and it is the content of the Quran that bears the substance of the Islamic call. Of course the fact that the Quran is alone the source of Islamic religious thought does not eliminate other sources and origins of correct thinking. As shall be explained later on, the Quran itself provides clarifications on other sources.
2. Ways Shown by the Quran for Religious Thought
There are three methods of religious thought in Islam. The Holy Quran in its teachings points to three paths for Muslims to follow in order to comprehend the purposes of religion and the Islamic sciences:
A) The External and Formal Aspect of Religion
It can be seen that the Holy Quran in its formal aspect addresses all people without providing any demonstration or proof. Rather, depending on the unique sovereignty of God, it commands people to accept the principles of faith such as divine unity, prophethood, eschatology; it gives them practical injunctions such as the daily prayers, fasting, etc.; and at the same time it prohibits them from committing certain other actions. Yet if the Quran had not provided authority for these commands it would never have expected man to accept and obey them. It must, therefore, be said that such simple utterances of the Quran are a path toward the understanding of ultimate religious ends and the comprehension of the Islamic sciences.
Indeed, the term "the external aspects of the religion" does not refer exclusively to the Quranic verses; rather it also includes sayings, conducts and assertions (silence) of Prophet Muhammad (pbuh) and considers them to be authoritative as indicated by the evident meaning of the Quranic verses. The Quran says:
"لقد کان لکم فی رسول الله اسوة حسنة"
[Certainly you have in the Messenger of Allah an excellent exemplar for him.]
Also, it has been proved through definite and profusely reported narrations from the Holy Prophet (pbuh) that his family's sayings, conducts and silence are on a par with his own sayings, conducts and silence.
In Shi'ism hadiths transmitted through the companions are dealt with according to this principle: if they deal with the words and actions of the Prophet and do not contradict the hadiths of the Household of the Prophet, they are acceptable. If they contain only the views or opinions of the companions themselves and not those of the Prophet, they are not authoritative as sources for religious injunctions. In this respect the ruling of the companions is like the ruling of any other Muslim. In the same way, the companions themselves dealt with other companions in questions of Islamic law as they would with any Muslim, not as someone special.
B) Intellectual Reasoning
In addition to guidance in the external aspect of religion, we see that the Holy Quran in many verses guides man toward intellectual understanding. It invites man to meditate, contemplate and deliberate upon the signs of God in the macrocosm and the microcosm. In many of its words and utterances the Quran attests to the validity of intellectual proof and rational demonstration, that is, it does not claim that man should first accept the validity of the Islamic sciences and then through intellectual proofs justify these sciences. Rather, with complete confidence in the truth of its own position it proclaims that man should use his intellect to discover the truth of the Islamic sciences, and only then accept this truth. It does not tell people to have faith first and then adduce arguments in accordance with it.
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". 
Shi'ite Initiative in Islamic Philosophy and Kalam
Those who are acquainted with sayings and works of the companions of the Prophet 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.
It was during the early 3rd century that numerous philosophical writings were translated into Arabic from Greek, and through them the method of philosophical thought became known to the general public. However, philosophical work ceased to exist beyond the seventh century among Muslim majority in the Sunni world. Although after Ibn Rushd (Averros) philosophy disappeared in the Sunni world, it continued to live in Shi'ism. The effective factor beyond the continued existence of philosophical thought in Shi'ism is the scientific treasuries left from the Shi'ite noble predecessors. 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.
C) Intuition and Unveiling
The Holy Quran in subtle terms explains that all true religious science originates and comes from Divine Unity (tawhid) and the knowledge of God and His Attributes. The perfection of the knowledge of God belongs to those whom He has drawn from all places and elevated solely to Himself. It is these men who have forgotten themselves and all things and as a result of sincerity in obedience to God have been able to concentrate all their powers and energies upon the transcendent world. Their eyes have become illuminated through the vision of the light of the Pure Creator. With the eye of discernment they have seen the reality of things in the kingdom of heaven and earth, for through sincerity of obedience they have reached the station of certainty (yaqin). As a result of this certainty the kingdoms of heaven and earth and the immortal life of the eternal world have become revealed to them.
Deliberation upon the following holy verses illuminates fully this claim:
"و اعبد ربک حتی یاتیک الیقین"
[And serve your Lord until there comes to you that which is certain.]
"کلا لو تعلمون علم الیقین لترون الجحیم"
[Nay! if you had known with a certain knowledge, you should most certainly have seen the hell.]
"فمن کان یرجوا لقاء ربه فلیعمل عملاً صالحاً و لایشرک بعبادة ربه احداً"
[Therefore, whoever hopes to meet his Lord, he should do good deeds, and not join any one in the service of his Lord.]
Firstly, the Commander of the Faithful, Ali bin Abi Talib is known particularly for his eloquent exposition of gnostic truths and the stages of the spiritual life. His words in this domain comprise an inexhaustible treasury of wisdom.
Secondly, observance of Shari'ah standards in one's spiritual journey and in his behaviors, and compliance with Shari'ah rules in Shis gnosticim has been the most important factor guaranteeing protection of Shiite gnosticism from intellectual and moral deviations.
3. Shi'ite Religious Thought
What we have explained so far forms the basis of Shi'ite thought which includes so wide a range of beliefs and codes of practice that even mentioning their titles and headings are too long to be incorporated by this writing. For this reason, we will suffice to mentioning some of the titles and headings pertaining to the theological domain and leave a detailed discussion concerning Shi'ite thought in the ideological and jurisprudential domains for some other times:
- Belief in the existence of God is a principle common to all divine religions. Arguments are presented in different ways on this principle.
- The first stage of divine unity is essential unity followed by unity of attributes and actions. That is, God is one and has no partner; He is abstract and free of any intellectual and external composition. God's essence is characterized by all the attributes of perfection and free of any deficiencies. His attributes are not an addition to His essence. He does not need anyone or anything in doing His tasks. No being can help Him in any manner.
- The world has no lord and contriver other than God. Other managers like the angels carry out their duties only with the will and permission of God.
- Unity in worship is the principle common to all the heavenly Shari'ah, and the purpose of prophetic mission of all the prophets was to remind about and stress on this principle.
- Belief in the intercession (shafa'ah) and invocation (tawassol) and that according to the Quranic doctrines, these two are not opposed to divine unity nor are they considered to be shirk (polytheistic).
- Belief in rational good and rational evil and that the intellect perceives that God is free from all evil acts and that He commits no unjust actions.
Belief in rational good and evil has many benefits one of which is the belief in justice. Justice itself consists of many branches and subdivisions such as the justice of God (belief in God being Just opens up many big gates of knowledge to us and solves many ideological issues), the necessity of a Mujtahid, political and social rulers and prayer leaders, witnesses etc. being just…
- Belief in "al-Amr bayn al-amrayn" i.e. a median approach towards the issue of determinism and freedom.
- The divine will requires that in order for mankind to tread the path of perfection and to be guided towards the lofty goals of creation, God should send prophets, and not suffice to intellectual guidance only.
- The divine prophets should be immune to intentional and unintentional errors in receiving the revelation from God, in guarding it and in making possible its reaching the people.
- The prophets are immune to all sins and errors.
- Prophet Muhammad bin Abdullah, peace be upon him and his family, is the last of all divine prophets. He started his prophetic mission by challenging others with his eternal miracle (the Holy Quran).
- The religion of Islam is general and global, not regional and continental, or racial and tribal.
- The Prophet of Islam is the seal of prophets, his book is the last of all books and his religion is the final of all religions.
- The divine book of Muslims (the Quran) is free from all kinds of distortions and alterations. Nothing has been added to or omitted from it.
- The divine will extends to the fact that the Prophet (pbuh) introduces the Imam and leader after him and that he, by appointing Ali bin Abi Talib (a.s.) as his successor on different occasions, carried out this crucial duty.
- The duties of the Imam after the demise of the Prophet of Islam (a.s.) are as under: To explain the Quranic concepts and Shari'ah laws, to prevent the society from deviation, to answer religious and ideological questions, to carry out and establish justice and equity in the society, to safeguard the borders of Islam against the enemies.
- The Imam and successor of the Prophet (pbuh) must be scientifically and morally under a special divine attention and should be divinely trained and educated; that is, he like the Prophet (pbuh) must be infallible and immune to all kinds of errors and mistakes. For this reason, the Prophet's successor is discerned and appointed divinely and introduced through the Prophet (pbuh) or a previous Imam.
- The successors of the Prophet (pbuh) are twelve immaculate Imams or "twelve caliphs" as reported in Shi'ite and Sunni sources. The first of them is Ali bin Abi Talib and the last of them is Hazrat Hujjat bin Al-Hasan Al-Askari (Imam Mahdi) – may Allah hasten his reappearance.
- The names of the twelve holy Imams and successors of the Holy Prophet are the following:
1. Ali bin Abi Talib, 2. Hasan bin Ali, 3. Hussein bin Ali, 4. Ali bin Hussein, 5. Muhammad bin Ali, 6. Ja'far bin Muhammad, 7. Musa bin Ja'far, 8. Ali bin Musa, 9. Muhammad bin Ali, 10. Ali bin Muhammad, 11. Hasan bin Ali, 12. Imam Mahdi (peace and benediction of Allah be upon them).
- The twelfth Imam, Imam Mahdi, the promised son of Imam Hasan Askari was born in 255 A.H. in Samarra and he is alive as of today and waiting to reappear at God's order. During his occultation, a qualified jurist takes the position of leadership and serves as the leader of the Islamic government.
- Another feature of Shi'ism is the openness of the gate of Ijtihad. That is, according to the Shi'ite religion, the application of the general principles to minor issues and deduction of Shari'ah laws from the sources are not restricted to what the predecessors have understood; the gate of inferring Islamic laws from the sources is still open.
- According to traditions in Shi'ite and Sunni sources, the reappearance of a man from the family of the Holy Prophet (pbuh) at the end of time to establish justice and to help mankind reach real perfection is one of the indisputable Islamic beliefs. In fact, the reappearance of a savior is one of the beliefs of all the different faiths and religions of the world such as Christianity, Judaism, Zoroastrianism etc. 
- Belief in raj'ah (return) and that some people return to the world after death and before the afterlife. 
- Belief in life after death and that man receives the reward for his deeds in the Afterworld, and death is not the end of life; rather man is moved upon death from one world to another and that there is another abode between this world and the next which is called Intermediate World which has its particular life, bounties and punishment.
 - Al-Ahzab: 21
 - Nahl: 125
 - Al-Hijr: 99
 - At-Takathur: 5 & 6
 - Al-Kahf: 110
 - For further reading, see Shiite Islam by Allamah Tabatabai, pp.75-114.
 - For further studies, see Muhammad Taqi Mesbah Yazdi, Amuzesh Aqaid, p.136 and 137.
 - For further studies, see Muhammad Taqi Mesbah Yazdi, Amuzesh Aqaid, pp.162 – 167.
 - For further studies, refer to "al-amr bayn al-amrayn" index, question 58 (site: 294).
 - For further studies in this regard and for understanding the political thought of Shi'ism, refer to the articles written on "Wilayat Faqih" (governance of the jurist) on this website.
 - For further studies, see "Imam Mahdi According to Shi'ite", question 168 (site: 1375).
 - For further studies, read "The Concept of Return (raj'ah) and Its Details", question 247 (site: 1112).
 - For further studies, see Ja'far Subhani, Manshur-e Aqaid Imamiyah (An Exposition of the Shi'ite Beliefs).