What are the boundaries of existence and non-existence in Buddhism?

We find plenty of similar discussions in the Theravada tradition, cf. (for example) kv6.1 or kv5.5 kv5.5 https://suttacentral.net/kv5.5/en/aung-rhysdavids?lang=en In my answer to the following question https://buddhism.stackexchange.com/questions/45940/ I mention in passing that (fantasy) hobbits are non-existents. Why? Because they are not subject to cause and effect, because they do not momentarily disintegrate, and because they leave no trace on the world. 'Unreal' and 'non-existent', to me, are synonyms. This leads onto a thread of comments discussing the putative existence of hobbits, hypotheticals, and whether or not one can distinguish the existence of hobbits from, for example, chairs. My grounding in this is presentation of the divisions of the selfless in as found in volume three of Jam-yang-shay-pa’s (1648-1721) "Great Exposition of Tenets" (there is a commentary on this in Chapter 1 of Hopkins "Meditation on Emptiness"). While the text itself is a Mahayana text rooted in Madhyamaka, volume three enumerates some important aspects of Buddhist reality, primarily following Abhidharma stemming from the Sarvāstivāda. What do we find? The first is the division into that which is (skt: sat), and that which is not (skt: asat). The word that describes this dichotomy is 'reality' or 'truth', or 'existence' (skt: satya, pali: sacca) - but 'existence' here is not to be confused with 'bhava' - because permanents (Pali: nicca) are sat, while nicca are not bhava (Bhava are things - compounded, while nicca are absences, and are uncompounded. Both nicca (as absences) and things can be found, therefore they are 'sat'). If we deny nicca, then we deny cessations - if we deny cessations, we have denied the third noble truth. Classical examples of non-existents are a hare's horn, turtle hair, clothing made from turtle hair. Things like a reflection's laugh. Also, inherently existing phenomena, or a self-created god, or an omnipotent being, or permanent products, or souls. So, the questions are: Are fictional hobbits 'sat' or 'asat'? Are hypotheticals 'sat' or 'asat'? How about a chair, or an elephant? What about the constant, π (pi)? I might be wrong - and am happy to be corrected, but I would consider hypotheticals, hobbits, constants, abstract objects (platonic abstracts) to be **asat**. Why? Because they are fabrications, narratives, stories - the stories they belong to are **sat**, and the stories themselves can instruct and inform us, but the objects in those stories are **asat** : they do not exist, they are not true, they are not real. If we allow for hobbits to exist, and if our rationale for such allowance similarly pervades all non-existents then we must also allow for omnipotent, self-created gods to exist, and souls. If we do that, then we cannot differentiate between what is a noble truth and what is not, and Dharma no longer holds truth, but is merely another story. Non-existent things, being unreal, they are both selfless, and are uncompounded. Being uncompounded they are neither able to create causes nor suffer them: They are not dependent arisings (as they are not subject to cause and effect) but they are dependent designations. Am I wrong? How so? **Addendum** - My question is concerned with conventional truths. I'm not attempting to establish or discuss objective/intrinisc truths. - If we allow for (fantasy) hobbits and if such rationale similarly pervades all non-existents, then we must allow for souls (because our reasons for how hobbits can exist do not exclude how souls can exist). - If we have allowed for souls, we do not have the three marks of existence. Because: anatta - If we do not have the three marks, we do not have insight into anattā / anātman - If we do not have insight then we do not have the three higher trainings (tisikkhā/triśikṣā). - Without the three higher trainings there is no noble truth of the path. - Without the truth of the path there is no Buddha dharma. - Without Buddha dharma there is no refuge, nor is there the awakening of enlightenment. *(amended in light of a good point made by Yeshe Tenley below)*