How to Remove Distracting Thoughts (MN 20)

Siddhartha reached enlightenment by reflecting on the unworthiness of unskillful thoughts. This produces shame-born-of-self-respect (hiri). Considering the danger in such thoughts produces a fear-of-wrongdoing (ottappa), which is born out of respect for others.

Wisdom Quarterly translation (MN 20: Vitakkasanthana Sutra)

JETA'S GROVE, Savatthi (Ancient India) - Thus have I heard. In Anathapindika's Park, the Buddha addressed the monastics: "When pursuing the higher mind [the eight meditative attainments, namely, the eight jhanas] one should give attention to five signs [nimittas, explained in the commentary as the following five strategies]:

"The Legend of Buddha" film directed by S.S. Phalke, artwork by Kingdom Animasia, Manila, animated backgrounds by Chennai artists (desikids.co.uk).

1. "If when giving attention to some sign, and owing to that sign, there arise unskillful, unwholesome thoughts -- connected with desire, aversion, or delusion -- then instead give attention to some other sign connected with what is wholesome and skillful. By turning attention from one to the other then such unskillful, unwholesome thoughts will subside and be abandoned. Just as if a carpenter were to knock out a coarse peg with a new one...by turning from an unwholesome to a wholesome thought, the mind becomes steady, quiet, one-pointed, and concentrated [calm and collected, at ease and purified, appeased and strengthened].
2. "If although having turned one's attention the mind is still distracted by unskillful, unwholesome thoughts then one should examine the danger in those thoughts: 'Such thoughts, being unwholesome, are reprehensible and result in suffering!' This examination results in their abandonment. Just as if a beautiful youth fond of ornaments were to have the carcass of a snake, dog, or human hung around his or her neck would on that account become horrified, humiliated, and disgusted -- so too with the examination of the danger in unwholesome thoughts: One abandons them, and the mind becomes steady, quiet, one-pointed, and concentrated.
3. "If when examining the danger, the mind is still distracted by unskillful, unwholesome thoughts, then one should forget those thoughts and not give any attention to them. By doing so such thoughts subside and are abandoned. Then the mind becomes steady, quiet, one-pointed, and concentrated. Just as if one, not wanting to see what had come into view, were to either turn away or shut one's eyes...so too when one forgets and does not give attention to such thoughts...the mind becomes steady, quiet, one-pointed, and concentrated.
4. "If when forgetting and not giving attention, the mind is still distracted by unskillful, unwholesome thoughts, then one should [stop the cause of the thought by way of inquiry: tracing the thought to its cause, and that thought to its cause, and so on; this, according to the commentary, is what it means to] still the thought-formation of those thoughts. [As explained by Bhikkhu Bodhi from the commentary, "such inquiry brings about a slackening and eventually the cessation of the flow of unwholesome thought."] It is just as if one walking quickly were to consider, 'Why am I walking quickly? What if I walk slowly?' and by such consideration would walk slowly, or walking slowly might consider, 'Why am I walking slowly? What if I stand still?' and by such consideration would stand still, or standing still might consider, 'Why am I standing still? What if I sit?' and by such consideration would sit, or sitting might consider, 'Why am I sitting? What if I lie down?' and by such consideration might lie down. In each case one would substitute a gross activity for a more subtle one. In the same way, when stilling the thought-formation of those thoughts the mind becomes steady, quiet, one-pointed, and concentrated.
5. "If when giving attention in this way the mind is still distracted by unskillful, unwholesome thoughts then by clenching one's teeth and pressing the tongue against the roof of the mouth, one should overcome, restrain, and suppress [an unwholesome state of] mind with [a wholesome state of] mind. Just as if a strong person were to seize a weaker one by the head or shoulders and thereby overcome and suppress that person so too...the mind becomes steady, quiet, one-pointed, and concentrated.

"When giving attention to some sign and on account of it distracting thoughts arise that are unskillful and unwholesome, one should give attention to a thought connected with what is skillful and wholesome. Thereby the mind becomes steady, quiet, one-pointed, and concentrated. When examining the danger in those thoughts... When forgetting and not giving attention to those thoughts... When stilling the thought-formation of those thoughts... When overcoming them by force...the mind becomes steady, quiet, one-pointed, and concentrated.

"This person is then called a master of the courses of thought. This person will think whatever thought is wished and not think any thought not wished. One has cut off craving, thrown off the fetters, and by completely penetrating conceit has made an end of suffering" [become fully enlightened]. When the Buddha had spoken, the monastics were delighted with the instructions.

• See also Discourse 20 in The Middle Length Discourses of the Buddha (Wisdom Publications) by Bhikkhu Nanamoli and Bhikkhu Bodhi.