The Holy Qur'an which has urged people to think, not only pointed out the causes of wrong thinking, but has also specified the subjects which are fit to be pondered on and which can be used as sources of knowledge and information.
On the whole Islam is opposed to wasting energy on the question which cannot be investigated or even if they can be, they are not beneficial to man.
The Prophet of Islam has called vain the knowledge, the achieving of which is of no use and the lacking of which causes no harm, on the other hand, Islam has encouraged the knowledge of those subjects which are useful and open to research.
The Holy Qur'an suggests three subjects for useful and beneficial thinking: Nature, History, and Human Conscience.
In many verses scattered throughout the Holy Qur’an the natural objects such as the earth, the sky, the stars, the sun, the moon, the clouds, the rain, the movement of the wind, the sailing of boats on the sea, the plants, the animals and every thing that man may perceive in his surroundings, have been mentioned as subjects fit to think over deeply and to draw conclusion. As an example we quote here one Qur'anic verse:
"Say: Behold what is in the heavens and the earth." (Surah Yunus, 10:101)
There are so many verses in the Holy Qur'an which invite man to the study of the past peoples and describe such study as a source of knowledge. From the Holy Qur'an's point of view all developments of human history take place in accordance with systematic norms and laws. All historical events involving honour or disgrace, success or failure, good luck or bad luck have their definite and well-calculated rules. By knowing these rules and laws current history can be brought under control and can be turned to the advantage of the present generation. Here there is one verse as an example:
"Different traditions existed in the past. So travel across the land and see the fate of those who denied the revealed truth." (Surah Ale Imran, 3:137)
The Holy Qur'an mentions human conscience as a special source of knowledge. From the Qur'anic point of view the entire creation consists of Divine signs and is a key to the discovery of truth. The Qur'an describes the world outside man as 'horizons' and the world inside him as 'selves', and thus instills in him the special importance of human conscience. That is why the term 'horizons and selves' has gained currency in Islamic literature.1
There is a world famous sentence of the German philosopher Kant, which is inscribed on his tombstone:
"There are two things which greatly excite the admiration of man: one is the starry sky located over our head and the other is conscience located within us".