The Sūfī 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 and 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 ʿUlyān, an eminent Sūfī, 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.” The _nafs_ of Hallāj was seen running behind him in the shape of a dog; and other cases are recorded in which it appeared as a snake or a mouse. [Sidenote: Mortification.] Mortification of the _nafs_ is the chief work of devotion, and leads, directly or indirectly, to the contemplative life. All the Sheykhs are agreed that no disciple who neglects this duty will ever learn the rudiments of Sūfism. The principle of mortification is that the _nafs_ should be weaned from those things to which it is accustomed, that it should be encouraged to resist its passions, that its pride should be broken, and that it should be brought through suffering and tribulation to recognise the vileness of its original nature and the impurity of its actions. Concerning the outward methods of mortification, such as fasting, silence, and solitude, a great deal might be written, but we must now pass on to the higher ethical discipline which completes the Path. Self-mortification, as advanced Sūfīs understand it, is a moral transmutation of the inner man. When they say, “Die before ye die,” they do not mean to assert that the lower self can be essentially destroyed, but that it can and should be purged of its attributes, which are wholly evil. These attributes--ignorance, pride, envy, uncharitableness, etc.--are extinguished, and replaced by the opposite qualities, when the will is surrendered to God and when the mind is concentrated on Him. Therefore ‘dying to self’ is really ‘living in God.’ The mystical aspects of the doctrine thus stated will occupy a considerable part of the following chapters; here we are mainly interested in its ethical import. The Sūfī who has eradicated self-will is said, in technical language, to have reached the ‘stages’ of ‘acquiescence’ or ‘satisfaction’ (_ridā_) and ‘trust in God’ (_tawakkul_).