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 Quran 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."