By the Book is meant the external aspect of the verses of the Holy Quran...

By the Book is meant the external aspect of the verses of the Holy Quran; and by the Sunnah, hadith received from the Prophet and his revered Household. Traditions of the 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.

The Qur’an and Tradition The Book of God, the Holy Quran, is the principal source of every form of Islamic thought. It is the Quran which gives religious validity and authority to every other religious source in Islam. Therefore, it must be comprehensible to all. Moreover, the Quran describes itself as the light which illuminates all things. Also, it challenges men and requests them to ponder over its verses and observe that there are no disparities or contradictions in them.

It invites them to compose similar work, if they can, to replace it. It is clear that if the Holy Quran were not comprehensible to all there would be no place for such assertions. To say that the Quran is in itself comprehensible to all is not in any way contradictory to the previous assertion that the Prophet and his Household are religious authorities in the Islamic sciences, which sciences in reality are only elaborations of the content of the Quran.

For instance, in the part of the Islamic sciences which comprises the injunctions and laws of the Shari’ah, the Quran contains only the general principles. The clarification and elaboration of their details, such as the manner of accomplishing the daily prayers, fasting, exchanging merchandise, and in fact all acts of worship) ‘ibadat (and transactions (mu’amalat), can be achieved only by referring to the traditions of the Holy Prophet and his Household.

As for the other part of the Islamic sciences dealing with doctrines and ethical methods and practices, although their content and details can be comprehended by all, the understanding of their full meaning depends on accepting the method of the Household of the Prophet.