What are the Sources of Knowledge?

Source: Man and Universe –by: Ayatollah Murtaza Mutahhari - p. 185-186

From the viewpoint of Islam the sources of knowledge are: the natural signs or the signs existing in the world, man himself, history or the social events and episodes of the nations and the communities, reason or the self-evident principles, heart, in the sense of the illuminating and purifying organ and the written record left by the past people.
In many verses the Holy Qur'an has asked people to ponder over the nature of the heavens and the earth: "Behold what is in the heavens and the earth." (Surah Yunus, 10:101)
Similarly the Holy Qur'an has invited people to study the history of the past nations intelligently with a view to take lesson from it: "Have they not travelled in the land so that they may have hearts to understand and ears to hear!" (Surah al-Hajj, 22:46)
The Holy Qur'an believes in the reliability of reason as well as of self-evident truths. It bases its arguments on them and says: "Say: 'Had there been gods besides Allah, then surely both the heavens and the earth (i.e. the whole universe) would have been thrown into confusion'." (Surah al-Ambiya, 21:22)
"Allah has not chosen any son, nor is there any god along with Him. Otherwise each god would have surely championed which he has created and some of them would surely have overcome others. Glorified be Allah above all that they allege." (Surah al-Mu'minun, 23:91)
Similarly the Holy Qur'an regards the heart as the centre of Divine inspirations and intuitions. Every man can receive inspiration in accordance with his sincere devotion and his effort to keep this centre spiritually pure and active. The revelation of the Prophets is the highest degree of this kind of knowledge. The Holy Quran has repeatedly referred to the value of the pen and the book and on several occasions taken an oath by them: "Nun. By the pen and that which they write therewith." (Surah al-Qalam, 68:1)