In the heaven-hell concept, pious go to Swarga (heaven) and sinners go to Naraka (hell). However, if one has good/bad karma, the rebirth concept says that he/she will enjoy/suffer in their next births.
How can both these theories be true when they contradict each other?
You said, "sinners go to Naraka (Hell)", yeah true. But what happened to the "sinner" there? Scriptures say, they get tormented in ways that can't be imagined. Who is the Sinner then? Note that the sinner's body is dead now. Whats left is the soul, and that ONLY should pass on to other Lokas e.g Heaven or Hell. Soul = "Atma" + a Subtle body i.e Jeevatma. Now Can Jeevatma alone be tormented or pampered? NO!! Let us see why.
Jeeva and Atma are different. Atma is eternally blissful, with trait like Satta, Chitta, Ananda. Nothing can harm Atma in ANY way. Its has (or feels) no Pain, no sorrow, has no memory, or seeks happiness either.
Jiva on the other hand is physical self that takes birth, grows, FEELS (pain or pleasure), and at last dies. And while it exists, it usually accumulates Karma, creates memories or say expressions on the psychic, which may all goes into its Subtle body, that is mentioned above.
So the Atma which is covered with accumulated Karma (and sometimes memory too), is called Jeevatma. Actually, Karma is a state of being of Atma itself. Precisely, kind of its Unbalanced state. So "Jeevatma" can also NOT be harmed or pampered in ANYWAY!!
So what's the need of Hell or Heaven when you can't harm a sinner-soul, or pamper a saint-soul in return, when it passes into the Loka!!???
Here comes Rebirth into play.
Re-Birth is nothing by mapping of an Jeevatma with a Jeeva, in that respective Loka. Be it NarakLoka (Hell), SwargaLoka (Heaven) or MrityuLoka (Earth). So when a Jeevatma takes rebirth, it becomes ready to get what it deserves, because ONLY THROUGH some "assigned" body, a Jeevatma can receive the fruits of its accumulated Karma.
And So again, TO GET THE FRUITS OF HIS DEED ONE HAS TO TAKE A RE-BIRTH, whether in Hell, or Heaven or here on Earth. That is why its said that a soul is always SENTENCED to Life, rather than "Gifted" a Life. And to be free is to, break this rebirth cycle.
There the two concepts DO NOT contradict, rather go hand in hand actually. THIS IS THE GREATNESS OF THE WISDOM OF HINDUISM.
Until Moksa is attained, all beings have to go through an endless cycle of birth and death. Some say that if a person has committed an especially heinous sin, the jiva may find itself in hell for some time. But nothing is forever, except Brahman. After suffering in hell for the karmic deed that sent it there, it will be reborn. Likewise with heaven. But there are enough ways to suffer on this earthly plane without the need of hell.
Krishna says in The Gita (Ch. 14, verses 14 and 15) " If the embodied soul meets with death when sattva prevails, it goes to the spotless realms [heaven] of those who know the Highest. If the embodied soul meets with death when rajas prevails, it is born among those who are attached to action [men]; and if it meets with death when tamas prevails, it is born in the wombs of creatures devoid of reason [animals, etc.]." - There is no mention of 'hell'....
This has already been answered straightforwardly in Mṛgendra Āgama.
The fruits which are effected by the modes related to the vainayika and prakruta are: Through dharma – heaven; through jnana – liberation; through vairagya – absorption into the prakriti tattva; through aisvarya – unobstructed and unassailable state; through adharma – occurrence of birth in the categories of animals and others; through ajnāna – hell; through avairagya – bondage; through anaisvarya – obstructions, limitations. 10.30.
Regarding what Vainayika & Prakṛta are,
The modes (bhavas) pertaining to the bound souls are classified into three – samsiddhika, vainayika and prakruta. Owing to the meritorious and auspicious deeds such as the construction of a temple, tank and such others performed by good-minded persons, a well-illumined consciousness dawns on them, free from the limitation and imperfection caused by anava mala.
And,
Samsiddhika is the constant manifestation of such perfect consciousness in them, even after the cessation of their bodily existence, in the same way as it was present during the embodied state. Vainayika is that consciousness which gets illumined and vitalized through Guru’s instructions(upadesa), study of the Scriptures and the knowledge gained through experience and intuition. Such vainayika is earned through mind, word and deed. Prakruta is that consciousness which occasionally manifests in the mind like a dream while being in embodied state.
Therefore, hell is a result of ajñāna whereas a lower birth is a result of adharma.**
NOTE : The answer is base on knowledge which I understood from the scriptures, I can also be wrong.
Starting from Geeta :
Sin and Good both are like fetter ( बेडियाँ ), one of Iron and one of Gold, you need to cut them both.
Meaning : Sins and Goods are all needed to be accumulated and reacted upon, they can never simply vanish away or cut each other.
The Life-Death cycle : In the whole lifespan the atman has to go through 84lac Yonis, these yonis are define throught their Karmas, and the lifespan of Atman end when it becomes Parmatman. Its Karmas are cut through these 84lac Yonis.
How its Done : I will go with an example
Case 1 : A person died, went with his deeds up, his left pocket has 20k of bad karmas and right has 50k of good karmas, the difference is 30k good karmas. The 30k good karmas are awarded with life in heaven. In heaven he lives like a Devta and alloted under the 33 categories (Many confuse 33 categories of Devta to 33 crore Devta) of Devtas, till those 30k good karmas are spend up. After that, that atman has to take birth again in human form with 20k bad as well as 20k good karmas.
case 2 : This person dies in another life and this time gains 10k good karmas and 70k bad karmas, the difference being 60k bad karmas, for these bad karmas he is awarded with life in downgraded yonis ie. Dog, Fish even Trees, and sometime if his bad deeds were so dangerous( ie. raping, murdering ,butchering etc. more explained here) not to be overcomed by the downgraded yonis is send to Hell for cutting its bad karmas. After that, the person is left with 10k of bad as well as 10k of good karmas, and takes birth in human form.
Because of equal bad and good kamras he had in both cases, he will go thruogh same amount of pain and happiness all over his life as is said 'Happiness and Sorrow come in equal amount over the life.'
This might have given some idea, but one confusion, what about atman, when body is dead, atman feels nothing, its just a part of the supreme soul.
The body is made of Five Layers, whenever a human die it leaves its Fifth layer which is 'Anamaya Kosh', made of materialistic entities like bones and flesh. The remaining four layers still remain and contain the Atman and these four layered body goes through the course of cutting its karmas, to cut the bad karma it either takes Anamaya Kosh of animals and plants or send directly to hell, and to cut the good deeds send to heaven. These four layers have 'Manomaya Kosh', which remembers everything, therefore an Atman remembers all its previous lives and even the torments in the hell. The atman is the first layer which is covered by the four Koshas or Layers.
I think this might help explaining the duality of Heaven-Hell and Reincarnation of the same Atman, as I stated above I can also be wrong, the question is a very broad one and different people will have different point of view in it.
You say "How can both these theories be true when they contradict each other?"
They don't contradict each other. The reason they don't contradict each other, is because you spend some time in heaven or hell, before reincarnating back to earth. You spend some time in heaven or hell as a result of your good or bad deeds, and once they are exhausted, you return to earth.
Here is how various scriptures describe this process for both heaven and hell:
Manusmriti 12.20:
If he practises virtue for the most part, and vice only in a small degree, then, invested with those same material substances, he obtains happiness in heaven.—(20)
Srimad Bhagavatam, Canto 11, Chapter 10:
Text 23: If on earth one performs sacrifices for the satisfaction of the demigods, he goes to the heavenly planets, where, just like a demigod, he enjoys all of the heavenly pleasures he has earned by his performances.
Text 24: Having achieved the heavenly planets, the performer of ritualistic sacrifices travels in a glowing airplane, which he obtains as the result of his piety on earth. Being glorified by songs sung by the Gandharvas and dressed in wonderfully charming clothes, he enjoys life surrounded by heavenly goddesses.
Text 25: Accompanied by heavenly women, the enjoyer of the fruits of sacrifice goes on pleasure rides in a wonderful airplane, which is decorated with circles of tinkling bells and which flies wherever he desires. Being relaxed, comfortable and happy in the heavenly pleasure gardens, he does not consider that he is exhausting the fruits of his piety and will soon fall down to the mortal world.
Text 26: Until his pious results are used up, the performer of sacrifice enjoys life in the heavenly planets. When the pious results are exhausted, however, he falls down from the pleasure gardens of heaven, being moved against his desire by the force of eternal time.
So, what this means is that if you do a lot of good deeds, you experience great happiness in heaven, and then when most of the results of those good deeds are exhausted, you return to Earth, but in higher life forms like a rich man, or a holy man, etc.
Manusmriti 12.21:
If on the other hand he practises vice for the most part, and virtue only in a small degree,—then, ... he suffers the torments inflicted by Yama.—(21)
Manusmriti 12.16:
In the case of misbehaved persons, there is produced out of five constituents, another strong body, for the suffering of torments, after death.—(16)
What this means is that when very evil people die, they go to Naraka (Hell) and get a new body that is specifically created to experience and withstand tremendous torture.
After they have suffered, through this body, the torments inflicted by yama, those constituents become dissolved into each of those same material elements.—(17)
Once the Jiva has experienced the suffering in Hell with this body, the body is destroyed and he no longer experiences the suffering in Hell. What happens to him now? This verse specifically answers your question:
Manusmriti 12.52:
Persons who have committed the heinous offences, having passed, during several years, through dreadful hells, reach, after the expiation thereof, the following migratory states.—(54)
So after suffering in various hells, they are reborn as creatures on Earth or as other supernatural creatures like Rakshasas.
Another verse:
Yājñavalkya 3.206:
Having passed through most despised hells, by virtue of their grievous sins, and thus having their had Karma exhausted, those who had committed heinous offences become born again in the world.
And just like the case of heaven, most of the punishment is meted in heaven, but the residue is experienced on earth. A person returning to earth from hell is reborn as plants, insects, animals, low-caste, diseased, poor, etc. And finally when even the residue of the sins are exhausted, he is born into noble families.
