How is a living being/lifeform defined in Buddhism?

A sentient living being, based on the sutta below, is one with the five aggregates present. But this is just a convention, just as the convention of what we call a chariot.

The five aggregates are form, feeling (or sensation), perception, consciousness and mental formations.

From SN 5.10:

Just as, with an assemblage of parts,
The word 'chariot' is used,
So, when the aggregates are present,
There's the convention 'a being.'

In the Sacca-vibhanga Sutta: Discourse on The Analysis of the Truths to the question, "What is death? This was the answer given:

"What is death? It is the passing away of beings in the various classes of beings; the falling away, the breaking up, the disappearance, the death, making end of life, the breaking up of the aggregates, the laying down of the body. This is called death.”

You are concerned about what happens when a person dies. Of what I’ve read, this is what takes place. We have three types (some say four) of bodies. One is the physical body that we leave at the time of death. The other two are the two parts of the mental (astral) body. This never dies, and gets transformed into an energy form (that is beyond time and space) at the time of Parinibbana.

Vinnana is always out looking for nourishment for the continuation of life from existence to existence. The food that Vinnana gathers get stored as a cittaja force, or as kamma seeds. These in turn become food for the sustenance of the vinnana. I’d call this the astral body of ours.

This astral body of ours (you may call it the vinnana) is always conditioned by the sansaric habits that we acquire from one bhava to another bhava. Also mainly three types of Cravings (Asava) and temptations (Anusaya) that lie dormant in us until the conditions are right and ripe.

The three Asava (that are steeped in Avijja) are: Ditti-asava, Kamasava, and Bhava-asava. These are deep rooted attachments that we have for certain views, for sense pleasures, and for existance. As long as these factors are there we will forever go in this sansaric journey.

Once you get into the Path, from the Stream Entrant Stage onwards, and as you go upstream from one stage to another, the ‘asava’ will start to lessen their hold on this astral body, and eventually get reduced to nothing. The way to go about this is the Noble Eighfold Path.

Sansära (or samsara) = san + sära (meaning fruitful) = perception that “san” are good, fruitful. Thus one continues in the long rebirth process with the wrong perception that it is fruitful.

Sammä = san + mä (meaning eliminate) = eliminate or route out “san”. Thus Samma Ditthi is routing out the wrong views that keeps one bound to sansara.

Tthe significance of “san” is illustrated in the verse that Ven. Assaji delivered to Upatissa (Ven. Sariputta before ordaining):

“Ye dhamma hetu pabbava, te san hetun Thathagatho aha, Te san ca yo nirodho, evan vadi maha Samano”

Te = three, hetu = cause, nirodha = nir+uda = stop from arising
“All dhamma (in this world) arise due to causes arising from the three “san”s: raga, dosa, moha. The Buddha has shown how to eliminate those “san”s and thus stop dhamma from arising”.

Being can be classified as:

• has 5 aggregates or at least the mental subset
• has has Nama-Rupa if in the material realm or Nama in the non material realms
• has at least mind in the 6 sense faculties
• arisen through Dependent Origination (DO)
• subjected to the the 4 Noble Truths (4NT)
• out of the 6 elements at least has consciousness
• has Citta and Cetasika and in the material realms rupa
• nourished and sustained by the 4 nutriments

A plant also lives but is not a being as it does not meet the above. It does not have a mind (henceforth Citta and Cetasika doe not arise) or consciousness, not subjected DO or 4NT. Plants also need nutrients but not the 4 nutrients which sustain a being.

A being is born another being in one of the 31 planes of existence or becomes extinguished if you attain Nibbana. An animate being cannot become an inanimate object like a stone.

Perhaps a way to access if a something is alive is through birth, aging and death along with the above factors. Something that is born ages and dies. Plants also does the same so they are alive but not beings.

As asked in the question, what are the characteristics by which a thing is classified as 'living being' or 'non-living' ? How is a lifeform defined in Buddhism?

A living being 'breathes'. The Pali word is 'pāṇa'. The Pali states:

Idha, cunda, ekacco pāṇātipātī hoti luddo lohitapāṇi hatapahate niviṭṭho adayāpanno sab­ba pāṇa­bhū­tesu.

There is the case where a certain person takes life, is a hunter, bloody-handed, devoted to killing & slaying, showing no mercy to living beings.

Cunda Kammaraputta Sutta

This is how a 'living being' is defined.

I mean, like how humans consider themselves as living beings due to various facts such as: their experience in this world due to consciousness, their actions (kamma), etc.

No.

A "living being" (pāṇa­bhū­tesu) is not the same as "a being" ("satta"). A "satta" is merely an idea or view. Please refer to SN 23.2 and SN 5.10 for an explanation.

if a living being (pāṇa­bhū­tesu) perform kamma (actions), they develop ideas or view of 'self', which is the meaning of 'a being' ('satta').

Note: Not all living beings perform kamma. Only h*m* sapiens & possibly higher life forms perform kamma.

If lifeform is defined as something that has consciousness and capable of producing kamma, what things are conscious?

Most living beings have consciousness, even if it is very primitive. For example, a worm will wriggle & try to escape if you touch it. That a worm senses sense stimuli would classify it as a conscious living being.

Where as a tree may breathe (carbon dioxide) however a tree is probably not conscious because it cannot sense & react to sense stimuli.

Is a plant a living being/conscious?

I answered this question before I read it. A plant is probably not a living being in the sense of the word "pāṇa­bhū­tesu" because killing plants does not fall under the precept about killing.

However, plants are obviously considered to be life forms.

Is a stone living being/conscious?

Of course not because as stone does not breath or have consciousness.

Is the universe conscious? Is everything a lifeform in some way?

That question is too deep & outside the scope of Buddhism. There are forces in the universe such as homeostasis & gravity; which infer some degree of communication but this is outside of the scope of Buddhism because Buddhism is only about ending suffering.

Buddhism states life (vitality) is dependent on heat & heat is dependent on vitality (MN 43). So what exactly is "life"? How long or short is a piece of string?

Is rebirth possible only as a lifeform? (I mean, by rebirth, just a transfer of consciousness/experience subject to different conditions. Please correct me if I'm wrong)

In Buddhism, the word 'birth' ('jati') refers to the arising of 'self-view' or egoism. Please refer SN 22.81, for example, or SN 12.2, which defines 'birth' as the production of the view of 'a being' ('satta'). 'Re-birth' is psychological rather than physical.

There is the case where an uninstructed...person — assumes form to be a self. That assumption is a fabrication. Now what is the cause, what is the origination, what is the birth, what is the coming-into-existence of that fabrication? To an uninstructed, run-of-the-mill person, touched by that which is felt born of contact with ignorance, craving arises. That fabrication is born of that. SN 22.81

Can our next rebirth be as a stone/whatever, other than the 5 predefined possibilities (human, god, ghost, animal, hell) in Buddhism?

If you are humane, moral & non-violent, this is a human rebirth. If you are extraordinarily loving, wealthy or powerful, this is a godly rebirth. If you are addicted to craving, this is a ghost rebirth. If you are immoral, violent & unthinking, this is an animal rebirth. If you are suffering or angry, this is a hell rebirth. In one single hour, you can reborn many times, whenever the mind grasps or mistakes different emotions as 'self'.

The next time the mind thinks the thought "I", "me", "mine", 'self", "him", "her", "them","us", "your", "you" or "our", this is the next rebirth. The Buddha taught each new birth brings suffering.

Being conscious, having consciousness, feeling, certain intellect... is not what makes a being a living being, thought on Unconscious beings (asaññasatta), still considered as satta (beings) althought nāma (mind is very subtile), Like a empty glass would be considered as empty of liquid, yet still wet.

Sects have developed their certain views and for the most plant like is not considered as living being, but if we take a look at the suttas, Buddha did not spoke of a para-Samsara in the wheel of birth:

"Vaseññha, I will tell you step by step how it happens,

The classification of living things in this and other births.

Look at the grass and trees, although they are not aware,

This and the other have attributes peculiar to their births.

So also insects, like grasshoppers and ants

They have attributes peculiar to their births.

Look at the animals small and large

They have attributes peculiar to their births.

Look at the serpents with long backs going on their bellies,

They have attributes peculiar to their births.

Look at the fish too, who find food in the water.

They have attributes peculiar to their births.

Look at the birds flying through the air.

They have attributes peculiar to their births.

Although these have various attributes, at birth,

In humans various attributes are not evident at birth.

They are not in the hair, head, ears or eyes

Not in the mouth, nose, lips or eye-lashes

from: Vāseṭṭha Sutta: To the Brahmin Vasettha

In judging at least, there are indications that the Buddha left it to ones possibility to recognice it as such, which is on one hand seen in the fact that in certain rules dealing with live not only the factor "it must be a living being" but also "one needs to recognize it as living being" are needed to be present.

In regard of how careful one needs to observe the pointing on: "even if as small as an bedbug" shows not only matters of size but also the knowing "this could have life" when thinking on egg as criteria.

In the Metta-sutta, the Lord Buddha is said to have meantioned: sata (creatures), pāṇā (what has life/breath), bhūtā (being born) and also "seeking for birth", aside of size and visibility as objects of metta (goodwill)

So generally, rather to seek for food that does not cause suffering or to speculate that in this or that birth one becomes not just food at the end, is to use the fact that life needs food, and life always ends up as being just food, wiser to let all appearances of life one recognises be a force od saṃvega ("fear") and to go on an gain and use life best and proper to make suffering an end while being as harmless as possible: Puttamansa Sutta: A Son's Flesh

And seeing that to much answering might derive others here from certain food from it, its maybe also a good time to make a leave here for now and let others take birth as they like by do not derive them from possible taking even a birth followed by Saddha (refuge in the Gems and Sila) and end up in the Unbound, the deathlessness.

By practicing the four Jhanas (improper) one can become a Unconscious beings (asaññasatta) which is generally not so desired by the most.

Anumodana, and don't forget your duties in regard of your Gods and father and ancestors not today!

Specially dedicated for: On Being of Noble birth - Stream winner reborn as human

(Note: This is a gift of Dhamma and not meant for commercial purpose or other wordily gains)