Shukr is thankfulness to God, and the Qur'an gives it a sharper edge than the English word carries by making its opposite kufr, the same root that names unbelief. To cover over a benefit received and to cover over the reality of God are treated as one movement of the will, which is why ingratitude is not a minor fault in this vocabulary but the shape unbelief takes. "If you are grateful, I will surely increase you; but if you are ungrateful, My punishment is severe" (14:7).
The Qur'an calls Abraham "grateful for His favours" (16:121), observes that "few of My servants are grateful" (34:13), and repeatedly presents the created order as a series of benefits whose recognition is the beginning of faith. Devotional practice gives shukr specific forms: the formula al-hamdu lillah, the prostration of gratitude (sujud al-shukr) offered on receiving good news, and thanksgiving sacrifice.
Al-Ghazali's analysis, which became standard, has three parts: knowledge that the benefit came from the giver, a state of joy directed at the giver rather than at the gift, and the use of the benefit in the way the giver intended. The third part is what makes gratitude a matter of conduct rather than feeling, on this account a person is ungrateful for eyesight not by failing to feel thankful but by using it for what is forbidden. Sufi writers extended the analysis to gratitude for affliction, on the ground that the giver's wisdom is not limited to what the recipient can recognise as benefit, a position that others treated as belonging to the advanced rather than as a general obligation.