The Mathnawi of Jalaluddin Rumi

Explaining that the purity and simplicity of the tranquil soul are disturbed by thoughts, just as (when) you write or depict anything on the surface of a mirror, though you may (afterwards) obliterate it entirely, (yet) a mark and blemish will remain (on the mirror).

The face of the tranquil soul in the body suffers wounds inflicted by the nails of thought.
Know that evil thought is a poisonous nail: in (the case of) deep reflection it rends the face of the soul.
In order that he (the thinker) may loose the knot of a difficulty, he has put a golden spade into ordure.

560. Suppose the knot is loosed, O adept (thinker): 'tis (like) a tight knot on an empty purse.
Thou hast grown old in (the occupation of) loosing knots: suppose a few more knots are loosed (by thee, what then?).
The knot that is (fastened) tight on our throat is that thou shouldst know whether thou art vile or fortunate.
Solve this problem, if thou art a man: spend thy breath (life) on this, if thou hast the breath (spirit) of Adam (within thee).
Suppose thou knowest the definitions of (all) substances and accidents, (how shall it profit thee?): know the (true) definition of thyself, for this is indispensable.

565. When thou knowest the definition of thyself, flee from this definition, that thou mayst attain to Him who hath no definition, O sifter of dust.
(Thy) life has gone (to waste) in (the consideration of logical) predicate and subject: (thy) life, devoid of (spiritual) insight, has gone in (study of) what has been received by hearsay.
Every proof (that is) without a (spiritual) result and effect is vain: consider the (final) result of thyself!
Thou hast never perceived a Maker except by means of a thing made: thou art content with a syllogism.
The philosopher multiplies links (consisting) of (logical) proofs; on the other hand, the elect (the mystic) is contrary to him (in this respect).

570. The latter flees from the proof and from the veil (between himself and God): he has sunk his head in his bosom for the sake of (contemplating) the Object of the proof.
If to him (the philosopher) the smoke is a proof of the fire, to us (mystics) 'tis sweet (to be) in the fire without the smoke,
Especially this Fire which, through (our) nighness and fealty (to God), is nearer to us than the smoke.
Therefore 'tis black villainy to go (turn away) from the Soul (Reality) towards the smoke for the sake of (indulging) the phantasies (illusions) of the (animal) soul.

In explanation of the saying of the Prophet, on whom be peace, “There is no monkery in Islam.”

"Do not tear out thy feathers, but detach thy heart from (desire for) them, because (the existence of) the enemy is the necessary condition for (waging) this Holy War.

575. When there is no enemy, the Holy War is inconceivable; (if) thou hast no lust, there can be no obedience (to the Divine command).
There can be no self-restraint when thou hast no desire; when there is no adversary, what need for thy strength?
Hark, do not castrate thyself, do not become a monk; for chastity is in pawn to (depends on the existence of) lust.
Without (the existence of) sensuality 'tis impossible to forbid sensuality: heroism cannot be displayed against the dead.
He (God) hath said ‘Spend’: therefore earn something, since there can be no expenditure without an old (previously acquired) income.

580. Although He used (the word) Spend absolutely, (yet) read (it as meaning) ‘Earn, then spend.’
Similarly, since the King (God) has given the command ‘Refrain yourselves,’ there must be some desire from which thou shouldst avert thy face.
Hence (the command) ‘Eat ye’ is for the sake of the snare (temptation) of appetite; after that (comes) ‘Do not exceed’: that is temperance.