Know that thinking is needed to remember what is forgotten, and to reflect on future benefits, but thinking of that which is not fruitful and has no benefit can be harmful, and if excessive, it will exhaust the body.
A person should focus their thoughts on what is possible for them to achieve, and what they can attain of good deeds. To spent a large amount of time reflecting on becoming a ruler is clearly not beneficial, but to reflect on how you can earn a greater reward from Allah through your deeds, can bring you great benefit.
One should also reflect on their personal struggle against evil. Many sinners have reflected on the consequences of their actions and repented, thus earning reward from Allah, praised and exalted be He.
We should also contemplate the purpose of our life and our end. Know that success lies not just in action but in the intention behind the action, so we must be mindful of all that we do. Ibn Abbas, may Allah be pleased with him, said:
“Praying two rak’ah with contemplation is better than praying all night while the heart is heedless” (Recorded in Ibn Mubarak)
Umm al-Darda’, may Allah be pleased with her, was asked: “What was the best deed of Abu’l-Darda’?” She said:
“Contemplation and thoughtfulness.” (Recorded in Ibn Mubarak)