The Sufi teachers gradually built up a system of asceticism and moral culture which is founded on the fact that there is in man an element of evil – the lower or appetitive soul. This evil self, the seat of passion or lust, is called nafs; it may be considered broadly equivalent to ‘the flesh,’ and with its allies, the world and the devil, it constitutes the great obstacle to the attainment of union with God. The prophet said: “Thy worst enemy is thy nafs, which is between thy two sides.” I do not intend to discuss the various opinions as to its nature, but the proof of its materiality is too curious to be omitted. Mohammed ibn ‘Ulyan, an eminent Sufi, relates that one day something like a young fox came forth from his throat, and God caused him to know that it was his nafs. He trod on it, but it grew bigger at every kick that he gave it. He said: “Other things are destroyed by pain and blows: why dost thou increase?” “Because I was created perverse,” it replied; “What is pain to other things is pleasure to me, and their pleasure is my pain.”
– Reynold Nicholson (1914, P 89-90). The Mystics of Islam.