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In Islamic law, najis (Arabic: نجس) are things or persons regarded as ritually unclean. According to Islam, there are two kinds of najis: the essential najis which cannot be cleaned and the unessential najis which become najis while in contact with another najis.
Contact with najis things brings a Muslim into a state of ritual impurity (Arabic: نجاسة najāsa, in opposition to ṭahārah, ritual purity). Ritual purification is then required before religious duties such as regular prayers are performed.
According to the Shafi'i school of Sunni Islamic jurisprudence, as systematised by al-Nawawi in his book Minhadj, the following things are najis: wine and other spirituous drinks, dogs, swine, dead animals that were not ritually slaughtered, blood, excrements, and milk of animals whose meat Muslims are not allowed to eat. Spirituous drinks are not impure according to the Hanafi school, while living swine and dogs are not impure according to the Malikis.
Additionally, any meat of animal which is killed in a manner other than that prescribed by Islam is najis.
Najis things cannot be purified, in contrast to things which are defiled only (mutanajis), with the exception of wine, which becomes pure when made into vinegar, and of hides, which are purified by tanning.
The Muṭahhirāt (The Purifying Agents)
It is possible to purify a thing which has become najis. These muṭahhirāt agents that can purify najis can be divided into three groups:
- The Qur'an says:
- “He (Allah) is the one who sends the winds as good news before His mercy; and We send down pure water from the cloud.” (25:48)
- According to the shari'ah, water can be of two types: muṭlaq and muḍāf. Muṭlaq is pure water without putting it to a scientific test. The five forms of muṭlaq are the following: rain, well water, running or flowing water (river or stream), Kur water (lake, sea or ocean), and less than kur. Muḍāf is mixed water as in tea or kool aid.
- the Earth
- the Sun
- istiḥālah (chemical change)
- inqilāb (change in properties)
- intiqāl (change in place)
- zawāl li-ʿayni n-najāsah (disappearance of the source of najāsah)
- istibrāʾ (quarantining)
- tabaʿīyyah (following)
- ghaybat al-muslim (absence of a Muslim)
Not all of these agents can purify every najis. However, among the agents water is the most universal purifying agent while the other agents are limited. 
Sources of law
The notions of ritual impurity come mainly from the Qur'an and hadith. Swine and blood are declared forbidden food in the Qur'an ([Quran 16:115], [Quran 6:145], [Quran 5:3], [Quran 2:173]). All this can be found in the Qurʾān