What are the psychological and metaphorical interpretations of the 6 realms and why are they significant to some Buddhists but not others?
The psychological reality of the 6 realms is real & not metaphorical. Where as the interpretations of the 6 realms as places that exist elsewhere, after the ending of life, is superstition. For example, when a vicious rapist or murderer or psychopath politician in the world is called "an animal", this is real & not a metaphor. They are animals & they are not human. In Buddhism, the word "human" ("manussa") means "high-minded" or "lofty-minded". The Pali suttas state a "human" lives in accordance with the Dhamma; with righteous conduct; without violence or exploitation ("devouring") of other beings. From the Buddhist point of view, not all people are "human". The book Buddha-Dhamma for Students may help here.
The real right understanding of the six realms is important to some Buddhists so they can see the world clearly (with dispassion), so they live the right way & so they can attain enlightenment. In other words, so they do not overestimate their spiritual progress & state. For example, many people who believe they are Buddhists & who are addicted to entertainment, sensuality, pornography, luxury, etc, believe the mind is "human" rather than the reality the mind is "hungry ghost". They believe by having superficial faith in the Buddha they will have a good reincarnation & don't realise the mind, in the here-&-now, is hungry ghost. This is how they overestimate their spiritual state.
Some Pali suttas listed below, which show the realms are mental states in the here-&-now.
I have seen, bhikkhus, the hell named ‘Contact’s Sixfold Base.’ There whatever form one sees with the eye is undesirable, never desirable; unlovely, never lovely; disagreeable, never agreeable. Whatever sound one hears with the ear … Whatever odour one smells with the nose … Whatever taste one savours with the tongue … Whatever tactile object one feels with the body … Whatever mental phenomenon one cognizes with the mind is undesirable, never desirable; unlovely, never lovely; disagreeable, never agreeable.
I have seen, bhikkhus, the heaven named ‘Contact’s Sixfold Base.’ There whatever form one sees with the eye is desirable, never undesirable; lovely, never unlovely; agreeable, never disagreeable. Whatever sound one hears with the ear … Whatever odour one smells with the nose … Whatever taste one savours with the tongue … Whatever tactile object one feels with the body … Whatever mental phenomenon one cognizes with the mind is desirable, never undesirable; lovely, never unlovely; agreeable, never disagreeable.
How has a monk attained/arrived at the state of a god?... A monk enters & dwells is the first...second...third...fourth jhana...
How has a monk attained/arrived at the state of a brahma?... A monk pervades the entire world with loving-kindness... compassion... appreciative joy... equinimity...
Living with Brahma are those families where, within the home, mother and father are respected by their children. Living with the early devas (gods) are those families where, within the home, mother and father are respected by their children. For what reason? Because mother and father are very helpful to their children, they take care of them and bring them up and teach them about the world.
Mother and father are called "Brahma," "early teachers" And "worthy of veneration," Being compassionate towards Their family of children.
Bhikkhus, a god, a human or any other good state would not be evident from actions born of greed, hate and delusion. Yet, bhikkhus, from actions born of greed, hate and delusion a hellish being, an animal birth a ghostly birth or some other bad state would be evident.
Bhikkhus, suppose a man would throw a yoke with a single hole into the great ocean, and there was a blind turtle which would come to the surface once every hundred years. What do you think, bhikkhus, would that blind turtle, coming to the surface once every hundred years, insert its neck into that yoke with a single hole?
If it would ever do so, venerable sir, it would be only after a very long time.
Sooner, I say, would that blind turtle, coming to the surface once every hundred years, insert its neck into that yoke with a single hole than the fool who has gone once to the nether world would regain the human state. For what reason? Because here, bhikkhus, there is no conduct guided by the Dhamma, no righteous conduct, no wholesome activity, no meritorious activity. Here there prevails mutual devouring, the devouring of the weak. For what reason? Because, bhikkhus, they have not seen the Four Noble Truths. What four? The noble truth of suffering … the noble truth of the way leading to the cessation of suffering.
Now on that occasion the wanderers of other persuasions had come together in a gathering and were sitting, discussing many kinds of bestial (animal) topics, making a great noise and racket. AN 10.93
Bhikkhus, these two bright principles protect the world. What are the two? Shame and fear of wrongdoing. If, bhikkhus, these two bright principles did not protect the world, there would not be discerned respect for mother or maternal aunt or maternal uncle’s wife or a teacher’s wife or the wives of other honored persons, and the world would have fallen into promiscuity, as with goats, sheep, chickens, pigs, dogs, and jackals. But as these two bright principles protect the world, there is discerned respect for mother… and the wives of other honored persons. AN 2.9
Beings (sattā) are few who, when they pass away from the animal realm (tiracchānayoniyā) are 'reborn' (paccājāyanti) among humans (manussesu)… those beings are more numerous are reborn in hell…. For what reason? They have not realised the Four Noble Truths. SN 56.102 to 131
Householders, there are these four ways of living together. What four?
A wretch lives together with a wretch; a wretch lives together with a female deva (god); a deva (god) lives together with a wretch; a deva (god) lives together with a female deva (god).
When both are immoral, miserly and abusive, husband and wife live together as wretches.
The husband is immoral, miserly and abusive, but his wife is virtuous, charitable, generous. She is a female deva living with a wretched husband.
The husband is virtuous, charitable, generous, but his wife is immoral, miserly and abusive. She is a wretch living with a deva husband.
Both husband and wife are endowed with faith, charitable and self-controlled, living their lives righteously, addressing each other with pleasant words.
Then many benefits accrue to them and they dwell at ease. Their enemies are saddened when both are the same in virtue.
Having practiced the Dhamma here, the same in virtuous behavior and observances, delighting in a deva world, they rejoice, enjoying sensual pleasures.
And what is the origination of the world? Dependent on the eye & forms there arises eye-consciousness. The meeting of the three is contact. From contact as a requisite condition comes feeling. From feeling as a requisite condition comes craving. From craving as a requisite condition comes clinging. From clinging as a requisite condition comes becoming. From becoming as a requisite condition comes birth. From birth as a requisite condition, then aging & death, sorrow, lamentation, pain, distress, & despair come into play. This is the origination of the world. SN 12.44
Here, Udayi, the bhikkhu secluded from sensual desires and thoughts of demerit abides in the first jhana: Overcoming thoughts and thought processes and the mind in one point internally appeased, without thoughts and thought processes abides in the second jhana. Again with equanimity to joy and detachment, feeling pleasant with the body too, abides in the third jhana. To this the noble ones say abiding in pleasantness with equanimity. Udayi, this is the course of actions, for realising the world of only pleasant feelings (ekantasukhassa lokassa). MN 79
Sensual desire… ill-will (anger)… sloth & drowsiness… restlessness & anxiety… uncertainty is an obstacle, a hindrance that overwhelms the mind and weakens wisdom… when a monk has not abandoned these five obstacles… for him to understand what is for his own benefit, to understand what is for the benefit of others, to understand what is for the benefit of both, to realize a superior human (manussa) state (dhammā), a truly noble distinction in knowledge & vision: that is impossible. AN 5.51
Six realms are mental states those experience even by humans. E.g Human being act like animals(only behave as responds to instincts) You are correct; realms can be used to describe both mental states and personalities of individuals.
See my interpretation and examples in this topic: My explanation of six realms of existence
And be note that six realms can't be divided by standard line. The characteristics are mixed gradually with other realms.That's why it is so difficult to analyze individual. But when human society is examined carefully behavior of six realms can be seen clearly.
To answer the question of "why are they significant to some Buddhists but not others?", I would say that some Buddhists are not interested in the psychological and metaphorical interpretations of the six realms, because they take the six realms to be real places that beings are reborn into.
And why do these Buddhists assume the six realms are literally existent places? They assume so because the Buddha said so.
In the Janussonin Sutta, Janussonin asks the Buddha whether deceased relatives would receive the gifts (I guess food) offered to them? The Buddha says that deceased relatives would not receive the gifts offered to them, if they were reborn in non-hungry-shade realms.
To quote an example:
"There is the case, brahman, where a certain person takes life, takes what is not given, engages in sensual misconduct, engages in false speech, engages in divisive speech, engages in abusive speech, engages in idle chatter, is covetous, bears ill will, and has wrong views. With the break-up of the body, after death, he reappears in hell. He lives there, he remains there, by means of whatever is the food of hell-beings. This is an impossible place for that gift to accrue to one staying there.
Above, it clearly says, "with the break-up of the body, after death, he reappears in ...". This is very unequivocal that someone's body breaks up and then he is reborn somewhere else in another realm.
As for hungry shades or hungry ghosts (peta):
"Then there is the case where a certain person takes life, takes what is not given, engages in sensual misconduct, engages in false speech, engages in divisive speech, engages in abusive speech, engages in idle chatter, is covetous, bears ill will, and has wrong views. With the break-up of the body, after death, he reappears in the realms of the hungry shades. He lives there, he remains there, by means of whatever is the food of hungry shades. He lives there, he remains that, by means of whatever his friends or relatives give in dedication to him. This is the possible place for that gift to accrue to one staying there.
Ven. Narada Mahathera wrote here:
Peta-yoni (pa + ita) lit., departed beings, or those absolutely devoid of happiness. They are not disembodied spirits of ghosts. They possess deformed physical forms of varying magnitude, generally invisible to the naked eye. They have no planes of their own, but live in forests, dirty surroundings, etc.
My personal hypothesis is that hungry shades are actually microbes in the environment. Hence, they are able to literally consume food offered to the deceased.
The gross psychological interpretations of the 6 realms are as follows: hate (hells), craving (pretas), stupidity (animals), passion (people), belligerance (asuras) and pride (gods, or devas). These are samsaric worlds, which means that even the worlds of gods are still mundane, not 'spiritual'.
That means that it is important to understand 'a realm level' for mundane purposes or for understanding another person's / one's own state of mind/existance, that is, at least from the first two Noble Truths, but it's irrelevant from the point of the Fourth Noble Truth (ayam dukkha-nirodha-gamini patipada).
These are of Learning, Realisation, Boddhisattvas and Buddhahood. As milestones on a path, they are important from the position of the Third and Fourth Noble Truths.
Because a human being can hardly encompass all the three thousand states at a time, it is not important from an [unadvanced] layman's perspective, since in terms of conventional logic we are capable of perceiving only a single state of mind within a given moment.