Shaykh Ibn Uthaymeen explains in his commentary, Sharh al-Mumti', that the foundational principle concerning water is that it is pure and purifying (tahur). This is based on the general meaning of texts from the Qur'an and Sunnah.

He clarifies that water is divided into two main categories, not three as some jurists have detailed:

1. Pure and Purifying Water (Al-Ma' al-Tahur): This is water that has remained in its natural state, created by Allah. It includes rainwater, seawater, river water, well water, and melted snow or hail. This type of water is used to perform wudu (ablution), ghusl (full bath), and to remove physical impurities from the body or clothing.

2. Impure Water (Al-Ma' al-Najis): This is water which was originally pure but has been mixed with an impure substance (najasah), causing a change in its taste, color, or smell. There is consensus (ijma') among scholars that such water is forbidden to use for purification.

Shaykh Ibn Uthaymeen addresses the third category, often termed 'pure but not purifying water' (al-ma' al-tahir ghayr al-mutahhir), which refers to water mixed with a pure substance (e.g., soap, saffron, or flowers) that alters its properties. He follows the opinion of Shaykh al-Islam Ibn Taymiyyah, arguing that this distinction is not based on strong evidence. The stronger view is that as long as the liquid is still referred to as 'water', it remains purifying, even if mixed with a pure substance. It only ceases to be purifying if the mixture changes its name entirely (e.g., it becomes 'soup' or 'juice') or if the change is caused by an impure substance.

Therefore, the default is that all water is pure and can be used for purification, unless it is altered by an impurity.