The Buddha was staying in Ujuñña, a town in the kingdom of Kosala, at a deer-park named Kannakatthale.1 He was approached by a man named Kassapa2, "the naked ascetic." He belonged to a different tradition, one which practiced extreme forms of asceticism - like wandering around in the nude, for example. As was customary, they exchanged polite greetings, and Kassapa stood to the side. "Master Gautama," he asked, "I have heard that you speak against all forms of self-mortification, and disparage those who practice these things. Is this true?"3
The Buddha answered, "No, Kassapa, this is not true. With the divine-eye, I am able to see one person who practices one kind of self-mortification being reborn into one of the hellish realms. In the same way, I can see another person who practices another kind of self-mortification being reborn into one of the heavenly realms. Even among those who do not practice their self-mortifications very earnestly, I can see those being reborn in these different destinations. Since I can see these beings arising and passing away like this, why would I disparage them all in the same way with some generalized blanket statement?"
He continued. "Kassapa, there are some ascetics and brahmins who are quite clever - but they love to argue. They learn the doctrines of their rivals just to split hairs in arguments. On some matters, I agree with these sorts of people. On others, I disagree. When I speak with them, I say, 'Let us set aside the points on which we disagree.' On points of agreement, wise people should use them as a basis for investigation and comparison of the different teachers and their communities.
"They should consider unskillful4 qualities - those things which should be abandoned, things which are unbefitting of noble people, and things which are dark - and ask themselves, 'which teacher has gone beyond these unskillful things - the ascetic Gautama, or these other teachers?' A wise person would praise me, saying, 'Gautama has gone beyond these unskillful things, while these other teachers haven't.'"
Here in the sutta, the Buddha goes on to say that wise people should consider skillful qualities in the same way, and declares that they would reach the conclusion that the Buddha, and not the rival teachers, has achieved those things. He also says that the sangha should be compared in the same ways to the commmunities of the other teachers.
"Kassapa," the Buddha continued, "I have taught a practice called the Noble Eightfold Path. If you follow it, you will see for yourself that my words are true and beneficial. You will not need to rely on hearsay. Namely, this path consists of: right view, right right thought, right speech, right action, right livelihood, right effort, right mindfulness, and right concentration.”
This part of the sutta consists of a long list of extreme ascetic practices, particularly associated with the Ājīvakās. The main idea here is that these are some of the more extreme forms of asceticism which the Buddha considered excessive; feel free to simply skim this part or skip to the next section.5
Kassapa said, “Master Gautama, the ascetics and brahmins who practice self-mortification consider these sorts of practices holy: they go around naked and ignore social etiquette; they lick their hands after eating; they do not listen when people ask them to come or stay put.
"There are many circumstances when they will not accept offerings of food—not when it is prepared for them, or when invited to a meal; they do not accept food which comes from pots and pans; they do not accept food from someone who keeps sheep, or from someone who owns a weapon or a shovel, or from a house where there is a couple eating, or from a house with a woman who is pregnant, breastfeeding, or staying with a man; they do not accept food from a house with a dog or with flies; they do not accept any meat or alcohol. They go to one house and take only one mouthful of food, or at most, seven houses for seven mouthfuls; they live on one small offering, eating once per day, or seven small offerings at seven points in the day, at most; they may eat once per day, or once every seven days, even reducing themselves to eating just twice a month.
"They take to eating only herbs, millet, raw rice or wild rice, water lettuce, rice husk powder, the scum produced from boiled rice, sesame flour, grass, or cow droppings. They may survive on forest roots and wild fruits, or the fruit which has fallen from trees.
"They wear robes made of rough or mixed hemp, shrouds taken from corpses, discarded rags, clothes made from tree bark, antelope hide, kusa grass, wood chips, cloaks of human hair, or hair from a horse’s tail, or owl’s wings. They pluck out their hair and beards. They refuse to sit, only allowing themselves to stand or squat. They lie on beds of thorns, or wooden planks, or in the mud. They only lie on one side, wearing filth like clothes, sleeping in the open air, wherever they happen to toss down their mat. They eat unnaturally, they don’t drink water, and they bathe themselves three times a day, to rid themselves of impurities."6
The Buddha said, “Kassapa, someone may practice all of these things, but if they haven’t developed and realized achievements in ethics, mind, and wisdom, they cannot truly be called an ascetic or Brahmin.7 If a monk develops a heart free of enmity and ill-will, filled with loving-kindess; if they abandon the corruptions and attain freedom through wisdom in this very lifetime, by way of the insight they gain through overcoming the corruptions… he can be called both a true ascetic and a Brahmin." Kassapa commented, "Master Gautama, truly, it is hard to be an ascetic and a Brahmin."
The Buddha replied, "Many people say so, Kassapa—but if one only needed to perform all of those austerities you mentioned to be a true ascetic and a Brahmin, it wouldn’t be right to say that it’s difficult. Truthfully speaking, any householder, or their son, or even a slave-girl who carries their water, could suddenly decide to start doing all of those austerities you mentioned.8 The reason it’s hard is because one needs to achieve something beyond those meaningless things—a higher sort of asceticism and Brahminism. A monk must develop a heart free of enmity and ill-will, filled with loving-kindess; if he abandons the corruptions and attains freedom through wisdom in this very lifetime, by way of the insight he gains through overcoming the corruptions… only then can he truly be called both an ascetic and a Brahmin."
Kassapa asked, "Master Gautama, how can someone become accomplished in ethics, mind, and wisdom in this way?"
The sutta presents the Sīlakkhandavagga Pericope as the Buddha’s response. The passages from "the arising of a Tathāgata" through "becoming perfected in the three types of ethics" corresponds to being "accomplished in ethics." The passages from "guarding the sense doors" through "attaining the 4 jhānas" corresponds to being "accomplished in mind." The passages from "attaining insight through samādhi" through "the corruptions are destroyed" corresponds to being "accomplished in wisdom."
"Kassapa," said the Buddha in conclusion, "there is no accomplishment higher than this."
The Buddha started speaking again. "Kassapa, some ascetics and Brahmins preach ethics - but when it comes to the highest, most noble ethical practice, I see no one who is my equal, much less my superior. Others preach self-mortification and austerity, but when it comes to the highest, most noble self-mortification and austerity, I see no one who is my equal, much less my superior." He repeated these statements regarding wisdom and liberation.
“Kassapa," he continued, "people from other traditions may speak ill of me, saying ‘That ascetic Gautama only lets loose his lion’s roar when he is alone, but he's silent when others are present.’ They should be corrected! They should be told, ‘Gautama lets loose his lion’s roar, even in the company of others, and he does so confidently!’”
The sutta continues keeps repeating this line, but each time, a new accusation is addressed, with the Buddha disputing it. It leads up to this final declaration made by the Buddha: "People who disparage me like that should be corrected! One should say: 'Gautama lets loose his lion’s roar in the company of others, confidently; people ask him questions, and he answers; they are satisfied with his responses, and they take it seriously; they’re inspired with confidence, and they express that confidence; they practice according to his teachings, and they see progress!'"
The Buddha began wrapping up his sermon to Kassapa. "Once, I was staying near Rājagaha, on Vulture's Peak.9 I was approached by a man named Nigrodhā, who practiced the lesser forms of self-mortification you and I have discussed today. He came to ask me about the higher forms of asceticism I have taught you. He was delighted to hear the truth which you have heard."
"Certainly, Lord!" Kassapa exclaimed. "Anyone who hears a teaching from you should be delighted. I am happy, myself, having been taught the truth. How splendid! Lord, I wish to be ordained in your order."
The Buddha replied, "Kassapa, since you are leaving another sect to join ours, we require that you wait for four months. After this period, if the sangha agrees, you may join our order." Kassapa excitedly said, "Lord, I would happily wait for four years, if at the end I could join your order!
The sutta concludes by telling us that Kassapa was later ordained by the Buddha personally, and soon after, he attained enlightenment while practicing meditation in solitude.
The corresponding sutra from the Chinese Canon is Dīrghāgama 25. I do not have access to an English translation of this sutra. I will update this section if I ever get the chance to read it.
 Walshe points out that the commentary elaborates on the deer-park - it was a public sanctuary where deer were declared off-limits from being hunted.
 Kassapa is an incredibly common name in the Buddhist texts. We saw another monk called Kassapa in DN 6. See this page for a detailed list of the figures who bore this name. The Kassapa at hand was "Acela-Kassapa," but even when being this specific, there is at least one other "Acela-Kassapa" in the scriptures, possibly two (sometimes it's hard to tell for certain if a scripture is referring a new person).
It is not mentioned explicitly in this sutta, but the Kassapa here must have been one of the Ājīvakās, students of Makkhali Gosāla. For more information, see my synopsis of DN 2.
 Here, "self-mortification" is translated from "tapam," which comes from "tapas" - heat. As best as I can gather, the reason this word is used in a religious/spiritual context is because it can refer to a type of spiritual energy produced through practice, or to the pain endured through extreme forms of self-punishment.
It is a general word which refers to a wide range of ascetic practices, but - as will be revealed in this sutta - Kassapa was clearly biased towards the most extreme forms of these practice. The Buddha spoke out against these forms of asceticism, but praised milder practices as the way to attain enlightenment. This is the idea of his "Middle Way" teaching.
Both Sujato and Walshe translate "tapam" as "self-mortification," in order to distinguish it from the more common terms which are translated as "asceticism," and I have followed their example for this synopsis.
 The word "unskillful" here is translated from "akusala." Where other religions would talk about "sinful" deeds and qualities, as opposed to those which are righteous, Buddhism instead discusses what is unskillful and what is skillful (kusala). Unskillful things are actions or thoughts which we commit because of our defiled minds. They are negative, and produce negative results, and it's because of this that they are considered bad. Skillful deeds, on the other hand, come from wholesome intentions and produce positive results. Morality in Buddhism can be understood as a matter of utility - whereas some religions might claim an action is "sinful" because it violates some divinely decreed law, Buddhism teaches that an action is "unskillful" because it makes things worse and produces negative results.
 In his translation, Walshe points out that this is a stock list that appears elsewhere in the suttas (for example, in MN 12.45).
 Regarding this point about ritual bathing, Walshe points to SN 7.2.11 for cross-reference. This sutta tells us of a man who lived in Sāvatthi, named Sangārava, who practiced this sort of ritual bathing. Ānanda requested that the Buddha visit him in order to teach him the Dhamma, and the sutta tells us that Sangārava converted after this visit.
 Walshe cites Rhys Davids in pointing out that the Buddha is using these terms "ascetic and Brahmin" in a very specific, subversive way. In covering previous suttas, I have touched on the way the Buddha often appropriated terms used by rival traditions and used them in new ways to strip them of their problematic contexts. Clearly, Kassapa had a very particular idea of what an "ascetic" is: someone who practiced all these extreme forms of austerity. Furthermore, the title "Brahmin" generally referred to a particular caste of society, especially the priests of the Vedic religion. The Buddha uses these terms, instead, to refer to someone who is accomplished according to his own teachings.
 In the full sutta, the Buddha is shown as repeating the entire list of extreme practices which were listed earlier by Kassapa. For the sake of brevity, I have simply shortened this to him saying "those austerities you mentioned."
 Rājagaha was the capital of the kingdom of Magadha in the Buddha's life. In later years, a newly-built city named Pātaliputtā became the capital. Regarding Vulture's Peak, it was an important location in the Buddha's life, and he and his monks often visited for retreat and meditation. It retained its importance in later Buddhist traditions, taking on new life as a mythical location where the Buddha delivered esoteric teachings to entire hosts of beings, natural and supernatural alike. In the Lotus Sutra, it's even depicted as a kind of afterlife, a "Pure Land" which survives the destruction of the other planes of existence. One way or another, the physical, real-world Vulture's Peak can still be visited today, in Bihar, India.