Ven. Nyanaponika’s research
5. Doubt
A. 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.
— SN 46:51
B. 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).
C. Simile
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.
— SN 46:55