Man has been created as a thoughtful being and the most important attribute that distinguishes him from other beings is the power of thinking.
However, thinking may not always be positive, constructive and beneficial. It could even cause one to deviate from the path that gets him to his goals.
So it is necessary that one knows the thoughts that are helpful and lead him to guidance and reflect on those thoughts.
In some verses of the Holy Quran God highlights issues that one needs to contemplate about. One of these issues is the world of creation.
“It is these who commemorate God while standing, sitting, or resting on their sides and who think about the creation of the heavens and the earth and say, ‘Lord, you have not created all this without reason. Glory be to you. Lord, save us from the torment of the fire.’” (Surah Al-Imran, Verse 191)
Another subject that is good to reflect upon is the history and fate of peoples who lived before us. Verse 21 of Surah Ghafir says: “Have they not travelled through the land to see the terrible end of those who lived before them. They had been mightier than them in power and in leaving their traces on earth. God punished them for their sins. They had no one to save them from God's torment.”
On the other hands, there are issues that we are advised not to think about because it would lead to problems and harms, such as thinking about the essence of God.