Shaikh Huseyin Hilmi Isik an Naqshbandi (rh) [d.1422AH,Istanbul,Turkey] wrote,"Muslims have to keep on learning from the cradle to the grave. The knowledge which Muslims have to learn is called al-'Ulum al-Islamiyya (Islamic sciences), which consist of two parts:

I) al-'Ulum an-naqliyya, II) al-'Ulum al-'aqliyya.

I) Al-'Ulum an-naqliyya (also called 'religious sciences'): These sciences are acquired by reading the books of the 'ulama' of the Ahl as-Sunnat. The 'ulama' of Islam derived these sciences from four main sources. These four sources are called al-adillat ash-Shariyya. They are al-Qur'an al-karim, al-Hadith ash-Sharif, ijma' al-Umma and qiyas al-fuqaha'.

Religious sciences consist of eight main branches:

1) 'ilm at-tafsir (the science of interpretation of Qur'an al-karim). A specialist in this branch is called a mufassir; he is a profoundly learned scholar able to understand what Allahu ta'ala means in His Word.

2) 'ilm al-usul al-hadith. This branch deals with classification of hadiths. Different kinds of hadiths are explained in Endless Bliss, second fascicle, sixth chapter.

3) 'ilm al-hadith. This branch studies minutely the sayings (hadith), behavior (sunnat), and manners (hals) of our Prophet (sall-Allahu 'alaihi wa sallam).

4) 'ilm al-usul al-kalam. This branch studies the methods by which 'ilm al-kalam is derived from al-Qur'an al-karim and al-Hadith ash-Sharif.

5) 'ilm al-kalam. This branch covers the study of the kalimat at-tawhid and the kalimat ash-shahada and the six fundamentals of iman, which depend on them. These are the teachings to be believed in by heart. Scholars of kalam usually wrote 'ilm al-usul al-kalam and 'ilm al-kalam together. Therefore, the layman takes these two branches of knowledge as one single branch.

6) 'ilm al-usul al-fiqh. This branch studies the derivation of the methods of fiqh from Qur'an al-karim and Hadith ash-Sharif.

7) 'ilm al-fiqh. This branch studies afal al-mukallafin, that is, it tells how those who are sane and pubescent should act on matters concerning the body. This is the knowledge necessary for the body. Afal al-mukallafin has eight sections: fard, wajib, sunnat, mustahab, mubah, haram, makruh and mufsid. However, they can be briefly classified into three groups: actions commanded, actions prohibited and actions permitted (mubah).

8) 'ilm at-tasawwuf. This branch is also called 'ilm al-ahlak (ethics). It explains not only the things we should do and we should not do with the heart but also helps the belief to be heartfelt, makes it easy for Muslims to carry out their duties as taught in 'ilm al-fiqh and helps one attain marifa.

It is fard-i 'ain for every Muslim, man or woman, to learn kalam, fiqh and tasawwuf as much as necessary out of these eight branches, and it is a guilt, a sin, not to learn them.

II) Al-'Ulum al-aqliyya (also called 'experimental sciences'): These sciences are divided into two groups: technical sciences and literary sciences. It is fard kifaya for Muslims to learn these sciences. As for Islamic sciences, it is fard 'ain to learn as much as is necessary. To learn more than is necessary, that is, to become specialized in Islamic sciences is fard kifaya. If there is no alim who knows these sciences in a town, all of its inhabitants and government authorities will be sinful.

Religious teachings do not change in the course of time. Making a mistake or erring while commenting on 'ilm al-kalam is not an excuse but a crime. In matters pertaining to fiqh, the variations and facilities shown by Islam can be utilized when one has the excuses shown by Islam. It is never permissible to make alterations or to make reforms in religious matters with one's own opinion or point of view. It causes one to go out of Islam. Change, improvement and progress in al-'Ulum al-'aqliyya are permissible. It is necessary to develop them by searching, finding and even by learning them from non-Muslim, too."