I think there is no good reason to think this sutta is not authentic. But to interpret it properly, it is good to keep in mind that, as @cdpatton pointed out, “early Buddhists tried to not get stuck with just one definition of things to avoid too much dogmatism.” In other words, don’t read this passage as strict definitions. Let me explain.
(Sorry, @Jayarava, for talking about you in the third person below. It felt easier to phrase things this way, being a response to external essays, not posts on this forum. I hope you don’t mind. Also, I hope you can take some criticism. )
While the word rūpa in its most general usage does not refer just to the body, it does certainly include it. Rūpa is detailed as the four elements, and in MN28 the internal four elements are exactly the parts of the body: “head-hairs, body-hairs, nails, teeth, skin, flesh, sinews, bones, bone-marrow, kidneys, heart, liver, …”, “bile, phlegm, pus, blood, sweat, fat, …” and so forth. When the Khajjanīya Sutta (SN22.79) says rūpa is ‘harmed’ or ‘hurt’ (ruppati) by cold, heat, hunger, thirst, and so forth, surely it refers to these “internal elements”. So in this text rūpa does refer to the body. (Though Jayarava is right that this doesn’t mean that rūpa therefore always means the body.)
Rūpa and ruppati are not etymologically related, but this isn’t problematic, because there is clearly a pun here, as Venerable Bodhi notes. This does not make the sutta inauthentic or otherwise untrustworthy. It just means we shouldn’t take this as a literal definition. That’s not something unique: word plays are all over the canon. As Jayarava acknowledges, Buddhism isn’t metaphysics, and this means the Buddha wasn’t that concerned about giving exact definitions. His first and foremost desire was to teach things that are practical, and sometimes he did so by being very non-literal. I would suggest that when he says rūpa gets hurt (ruppati), he is primarily reminding us of the suffering of the body.
I think the explanations of some of the other khandhas in the Khajjanīya Sutta are also not meant as literal definitions, including vedanā and particularly saṅkhāra, which elsewhere is more standardly defined as cetana ‘intention’ (SN22.56). The questions such as “why do you call it form?” are not how definitions are phrased elsewhere, suggesting that something different is going on here. The questions seem aimed more towards answering “why do we use this word?” rather than “what does this word exactly mean?”
Jayarava, expecting literalism, demands that in order for the “definition” of rūpa to make sense it must use a denominative verb rūpayati/rūpeti instead of ruppati, but at the same time he also acknowledges that such a verb doesn’t occur in the Nikāyas. It is very plausible there simply was no such verb in use at the Buddha’s time, which would explain why he chose to creatively use ruppati instead. You can’t expect someone to use a verb if that verb didn’t exist in the ordinary language. That would be akin to demanding someone to use the word ‘wood’ or ‘money’ as a verb.
So all in all it is a very weak basis for questioning this sutta, which @Khemarato.bhikkhu already pointed out contains no significant other indications of being inauthentic. I feel the puns actually make this sutta more likely to be authentic (or this section at least) because later additions trend towards systematic standardization and classification, not creative use of words such as this. That ruppati also isn’t simply a scribal mistake for rupeti is clear from the way the sutta explains it. “Rūpa is harmed by hunger/mosquitos” makes no sense if read as “rūpa ‘appears’ by hunger/mosquitos”. Yet the essay basically suggests the latter reading and blames others for misreading.
The word rūpa indeed has lots of connotations, one of which is indeed ‘appearance’. Shapes or colors you see with the eyes come under rūpa, for example. It also includes appearances which are perceived with the mind. MN28 for instance says that mind-contact can lead to rūpa, and DN33 says that there is rūpa that is visible and “resistant” (i.e. tangible matter) but also rūpa that is invisible [to the eye] and intangible. This might seem strange to us, but that’s because of our modern worldview which separates the mind from matter. Many ancients didn’t think like that. For example, some of the Greek stoics who lived around the time of the Buddha thought the soul lived on in the fire element. The four elements are not just external things but also their properties, and these properties extend into the mental realm as well. And therefore so too does rūpa.
But more generally rūpa definitely includes physical things as well, such as the water in the ocean or the air in the sky (MN28) or indeed the body. Because included within things that have an appearance are our bodies. To illustrate, in SN22.100 a painter draws the form (rūpa) of a person. And when you draw a person, you draw their body, not their mind or other internal aspects of their experience. The Bhrdaranyaka Upanishad has the line: “He is so-and-so by name [nāma] and has such-and-such an appearance [rūpa].” Here rūpa also effectively refers to the person’s body. In DN15 nāmarūpa is said to take shape in the mother’s womb, forming the embryo, where again rūpa refers (primarily) to the body.
To conclude, the term rūpa is impossible to strictly define, because it is used in different ways, and because concepts of “matter” were different back then. Many suttas have broader uses of rūpa, but in some the physical aspects are emphasized. Rūpa is not exactly identical to the body, but in certain contexts the connection with the body is so strong that to translate it as ‘body’ would not be completely unwarranted. This is also the case for the aggregate of rūpa in various suttas, although you couldn’t get away with that in the canon as a whole, which is why most translators stick to ‘form’.
The essay on vedanā has me baffled, particularly its attempted derivation of vedayati/vedeti. The verb is not a causative derivation. It’s simply a seventh conjugation from the root vid. So it just means ‘to experience/feel’, not ‘to cause to experience/feel’. Therefore the noun vedanā is also not derived from a causative.
Ved- is also not a “causative form of the root”. It’s a vuddhi (lengthening) of the root, which by itself does not make for a causative. The causative is determined by the suffixes added to the root. The Sanskritic form vediyati has no such suffixes and therefore has nothing to do with the causative. Vetter is right in noting that it is just a variant of vedayati. It is an alternative manuscript spelling not just in this sutta but throughout the canon.
I don’t know how these mistakes happened. Any serious Pali student should know what constitutes a causative, and should definitely know about the 7th conjugation. It’s in lesson three of Warder’s Introduction to Pali, for example. And Warder even specifically lists a vedeti as a 7th conjugation, as does Duroiselle. That these things are missed, plus other grammatical mistakes later on, strongly puts into question the credibility of this essay.
Now, probably what confused Jayarava is that the causative conjugation can sometimes result in the same stem as the 7th conjugation, when the causative suffix is -e. In such cases we have to consider the contexts the verb is used in order to tell whether it is a causative or not. And if we do, we’ll see that vedeti is (generally) not used as a causative in the Pali Canon. The PTS Pali-English Dictionary says it knows of “only one caus[ative]” application of vedeti. (emphasis in the original) It’s the aorist avedi in Jataka 444, where it has the same meaning as pavedeti ‘to inform’, which is semantically unrelated to vedanā. There may be some other instances which the dictionary missed, but they certainly would be outliers. In any of the common uses, those that relate directly to aggregate of vedanā, vedeti is not a causative.
The comparison of the five verbs in the Khajjanīya Sutta’s questionnaire on the khandhas also seems a strange way to determine meaning to me, and I don’t understand the conclusion at all. It translates a second person verb as a third person and a supposed denominative seemingly as a causative. It’s all rather senseless to me, so I agree, although for other reasons, that “all this etymological work […] is ultimately futile”. Regardless, if we do allow this strange route of argumentation, none of the other four verbs are a causative, so that would tell us that vedayati is not a causative either. (By the way, the reason only abhisaṅkharonti is plural and not the other four verbs, is simply that saṅkhāra is the only aggregate that is set in the plural.)
Vedanā is not the “positive (sukha), negative (dukkha), or neutral (asukham-adukkha) feelings that we have in response to sense experience”. It is the intrinsic pleasantness or unpleasantness of those experiences themselves. For example, sick people are said to have dukkha vedanā on account of their sickness, even if they were enlightened, as was the case for Maha Kassapa in SN46.14. The Buddha also had painful bodily feelings when he cut his foot in SN1.38.
So the critiques raised in these essays are unfounded. They’re based at least in part on bad understanding of Pali and on a failure to appreciate the non-technical use of language that’s quite common in the suttas. It blames Vetter and Hamiton of fixating on a single sutta, and that may be a rightful observation (I haven’t read those works) but itself overly fixates on etymology, which is even more problematic, especially if this etymology is incorrect.
I hope I didn’t word any of that too strongly. I do appreciate the inquiries Jayarava made. He does raise some very good points as well, especially some in the essay on saññā.