What is deep thinking?

One of the most important characteristics of those who believe in Allah is their ability to see the signs created by Allah. A believer sees the power and artistry of Allah in every subtlety around him, he glorifies His praise and takes this as a way to draw nearer to him. Believers having this characteristic are thus defined in the Qur’an:

Those who remember Allah, standing, sitting and lying on their sides, and reflect on the creation of the heavens and the earth: ‘Our Lord, You have not created this for nothing. Glory be to You! So safeguard us from the punishment of the Fire.’ (Surah Al ‘Imran, 191)

Allah emphasizes the importance of deep thinking to the believers in many places in the Qur’an with the phrases “Will you not reflect?”, “There are signs for those who reflect”. The subjects to be reflected on are unlimited. A believer ponders upon many subjects, like the extraordinary order in universe, the living beings on the earth, the perfect systems in these living beings, any event he experiences, the blessings Allah creates ceaselessly, disasters sent down to the unbelievers, Paradise, Hell, and eternity, and with profound thinking, he makes a better appraisal of the existence, might and infinite wisdom of Allah.

What is deep thinking?