Nourishment of Doubt:
There are things causing doubt; frequently giving unwise attention to them — that is the nourishment for the arising of doubt that has not yet arisen, and for the increase and strengthening of doubt that has already arisen. (source: SN46.51)
Denourishing of Doubt:
There are things which are wholesome or unwholesome, blameless or blameworthy, noble or low, and (other) contrasts of dark and bright; frequently giving wise attention to them — that is the denourishing of the arising of doubt that has not yet arisen, and of the increase and strengthening of doubt that has already arisen.
Of the six things conducive to the abandonment of doubt, the first three and the last two are identical with those given for restlessness and remorse. The fourth is as follows:
Firm conviction concerning the Buddha, Dhamma and Sangha.
In addition, the following are helpful in conquering Doubt:
Reflection, of the factors of absorption (jhananga);
Wisdom, of the spiritual faculties (indriya);
Investigation of reality, of the factors of enlightenment (bojjhanga). (source: The Five Mental Hindrances and Their Conquest, Nyanaponika Thera)
If there is a pot of water which is turbid, stirred up and muddy, and this pot is put into a dark place, then a man with a normal faculty of sight could not properly recognize and see the image of his own face. In the same way, when one's mind is possessed by doubt, overpowered by doubt, then one cannot properly see the escape from doubt which has arisen; then one does not properly understand one's own welfare, nor that of another, nor that of both
; and also texts memorized a long time ago do not come into one's mind, not to speak of those not memorized. (source: SN46.55)
A man travelling through a desert, aware that travellers may be plundered or killed by robbers, will, at the mere sound of a twig or a bird, become anxious and fearful, thinking: "The robbers have come!" He will go a few steps, and then out of fear, he will stop, and continue in such a manner all the way; or he may even turn back. Stopping more frequently than walking, only with toil and difficulty will he reach a place of safety, or he may not even reach it.
It is similar with one in whom doubt has arisen in regard to one of the eight objects of doubt. Doubting whether the Master is an Enlightened One or not, he cannot accept it in confidence, as a matter of trust. Unable to do so, he does not attain to the paths and fruits of sanctity. Thus, as the traveler in the desert is uncertain whether robbers are there or not, he produces in his mind, again and again, a state of wavering and vacillation
, a lack of decision, a state of anxiety; and thus he creates in himself an obstacle for reaching the safe ground of sanctity (ariya-bhumi). In that way, sceptical doubt is like traveling in a desert. (source: The Five Mental Hindrances and Their Conquest, Nyanaponika Thera)
The abandonment of sceptical doubt:
There is a strong man who, with his luggage in hand and well armed, travels through a wilderness in company. If robbers see him even from afar, they will take flight. Crossing safely the wilderness and reaching a place of safety, he will rejoice in his safe arrival. Similarly a monk, seeing that sceptical doubt is a cause of great harm, cultivates the six things that are its antidote, and gives up doubt. Just as that strong man, armed and in company, taking as little account of the robbers as of the grass on the ground, will safely come out of the wilderness to a safe place; similarly a monk, having crossed the wilderness of evil conduct, will finally reach the state of highest security, the deathless realm of Nibbana. Therefore the Blessed One compared the abandonment of sceptical doubt to reaching a place of safety.
(source: The Five Mental Hindrances and Their Conquest, Nyanaponika Thera)