The "purifier" is either water -which is, in this respect, basic or "earth". Know that the traveller to Allah has, generally speaking, two ways to take him to the loftiest goal, the station of proximity to the Divinity: The first of them, which is the principal and original one, is the journey to Allah by turning towards the Absolute Mercy, especially the compassionate mercy, which is the compassion that takes every being to its appropriate perfection. It is of this kind of compassionate mercy that the prophets (SA) were sent to lead on the roads and to help those lagging behind. To the people of knowledge and the people of heart, the House of Realization is the form of divine mercy. The creatures are perpetually and completely drowned in the oceans of Allah's mercy, yet they do not make use of it. The Great Divine Book -which has descended from the divine invisible world and the proximity of the Lord, and has appeared in the form of words and speech so that we, the deserted, the prisoners in the jail of nature and put in the fetters of the crooked chains of the soul's desires and whims, make use of it and rescue ourselves -is one of the greatest manifestations of the absolute divine mercy, of which, we, the blind and deaf, have in no way made use. The Messenger the Seal of the prophets, the honourable absolute guardian ,who came from the Sacred Presence of the Lord and the company of the divine proximity and familiarity to this abode of estrangement and dread, where he had to keep company with the people like Abu Jahl (the Prophet's uncle and his bitter enemy) or even worse, and whose sigh:" ...my heart is enveloped by a cover of dust... "  has burnt the hearts of the people of knowledge and friendship -is Allah's vast mercy and the divine absolute generosity, who had come into his (worldly) body as an all-embracing mercy for the dwellers of this lower world, in order to take them out of this abode of terror and estrangement, like a "ring-dove"  which throws itself into the net of blight to save its flock.
The traveller to Allah is to take the purification with the water of mercy as to be a form of using the descending divine mercy, and to make use of it as long as it is possible for him to do so. Should his hand become short of it, because of inertia or negligence, and be bereaved of the water of mercy, he would have but to pay attention to his own humility, indigence, poverty and destitution. When he has in full view his humility of servitude and is aware of his need, his poverty and his own potentiality, and discards his haughtiness, conceit and selfishness, a door of mercy opens to him, and the earth of (his) nature changes into the white earth of mercy, and becomes the dust which is one of the "two purifiers" , and becomes the object of Allah's mercy and kindness. The stronger this state in man, i.e., his awareness of his humility, the more he receives of mercy. Should he decide to depend on himself and on his action in his journey, he would perish, since there might be no one to extend help to him, like an infant which boldly starts walking alone, taking pride in its own steps, and depending on its own ability, without its father offering it any help, rather leaving it to itself. But when it recognizes its inability and inefficiency, it turns to its affectionate father, distrusting its own power, and entrusts itself to the care of its father, who offers his help, hugs it and guides it step by step to walking. So, it is better for the traveller to Allah to break the leg of his journey and completely renounce his self-confidence, austerity and action, and abolish himself, his power and ability. He should always remember his mortality and dependence in order to become an object of Allah's care, and to cover a hundred-year distant road in a single night by the attraction of the Lord, and the tongue of his inside and his state say, in the Presence of the Lord's Sanctity, imploringly and helplessly: "Or, Who responds to the distressed, when he calls unto Him, and removes the evil... ?" 
. Refer to footnote No. 88
. Kalilah wa Dimnah, ch. "The Ring-Dove".
. It refers to a narrative related by the late Akhund Khurasani (may Allah sanctify his spirit) in Kifayatul 'Usul, vol. 1, p. 130, to the effect that: "Dust is one of the two purifiers, and is enough for you for ten years."
. Surah an-Naml: 62.