"Nibbāna is bliss supreme."
Nibbāna is the summum bonum of Buddhism.
However clearly and descriptively one may write on this profound subject, however glowing may be the terms in which one attempts to describe its utter serenity, comprehension of Nibbāna is impossible by mere perusal of books. Nibbāna is not something to be set down in print, nor is it a subject to be grasped by intellect alone; it is a supramundane state (Lokuttara Dhamma) to be realized only by intuitive wisdom.
A purely intellectual comprehension of Nibbāna is impossible because it is not a matter to be arrived at by logical reasoning (atakkāvacara). The words of the Buddha are perfectly logical, but Nibbāna, the ultimate Goal of Buddhism, is beyond the scope of logic. Nevertheless, by reflecting on the positive and negative aspects of life, the logical conclusion emerges that in contradistinction to a conditioned phenomenal existence, there must exist a sorrowless, deathless, non-conditioned State.
The Jātaka Commentary relates that the Bodhisatta himself in his birth as the ascetic Sumedha contemplated thus:
"Even as, although Misery is,
Yet Happiness is also found,
So, though indeed Existence is,
Non-existence should be sought.
"Even as, although there may be Heat,
Yet grateful Cold is also found,
So, though the threefold Fire exists,
Likewise Nirvāna should be sought.
'Even as, although there Evil is,
That which is Good is also found,
So, though 'tis true that birth exists.
That which is not birth should be sought. "
The Pāli word Nibbāna (Samskrit--Nirvāna) is composed of "Ni" and "Vāna". Ni is a negative particle. Vāna means weaving or craving. This craving serves as a cord to connect one life with another.
"It is called Nibbāna in that it is a departure (Ni) from that craving which is called Vāna, lusting. "
As long as one is bound up by craving or attachment one accumulates fresh Kammic activities which must materialise in one form or other in the eternal cycle of birth and death. When all forms of craving are eradicated, reproductive Kammic forces cease to operate, and one attains Nibbāna, escaping the cycle of birth and death. The Buddhist conception of Deliverance is escape from the ever-recurring cycle of life and death and not merely an escape from sin and hell.
Nibbāna is also explained as the extinction of the fire of lust (lobha), hatred (dosa), and delusion (moha).
"The whole world is in flames," says the Buddha. "By what fire is it kindled? By the fire of lust, hatred and delusion, by the fire of birth, old age, death, sorrow, lamentation, pain, grief and despair is it kindled."
Nibbāna, in one sense, may be interpreted as the extinction of these flames. One must not thereby infer that Nibbāna is nothing but the extinction of these flames.  The means should be differentiated from the end. Here the extinction of the flames is the means of attaining Nibbāna.
Is Nibbāna Nothingness?
To say that Nibbāna is nothingness simply because one cannot perceive it with the five senses, is as illogical as to conclude that light does not exist simply because the blind do not see it. In a well-known fable the fish, who was acquainted only with water, arguing with the turtle, triumphantly concluded that there existed no land, because he received "No" to all his queries.
"Once upon a time there was a fish. And just because it was a fish, it had lived all its life in the water and knew nothing whatever about anything else but water. And one day as it swam about in the pond where all its days had been spent, it happened to meet a turtle of its acquaintance who had just come back from a little excursion on the land."
"Good day, Mr. Turtle!" said the fish. "I have not seen you for a long time. Where have you been?"
"Oh", said the turtle, "I have just been for a trip on dry land."
"On dry land!" exclaimed the fish. "What do you mean by on dry land? There is no dry land. I had never seen such a thing. Dry land is nothing."
"Well," said the turtle good-naturedly. "If you want to think so, of course you may; there is no one who can hinder you. But that's where I've been, all the same."
"Oh, come," said the fish. "Try to talk sense. Just tell me now what is this land of yours like? Is it all wet?"
"No, it is not wet," said the turtle.
"Is it nice and fresh and cool?" asked the fish.
"No, it is not nice and fresh and cool," the turtle replied.
"Is it clear so that light can come through it?"
"No, it is not clear. Light cannot come through it."
"Is it soft and yielding, so that I could move my fins about in it and push my nose through it?"
"No, it is not soft and yielding, You could not swim in it."
"Does it move or flow in streams?"
"No, it neither moves nor flows in streams?"
"Does it ever rise up into waves then, with white foams in them?" asked the fish, impatient at this string of Noes.
"No!" replied the turtle, truthfully, "It never rises up into waves that I have seen."
"There now," exclaimed the fish triumphantly. "Didn't I tell you that this land of yours was just nothing? I have just asked, and you have answered me that it is neither wet nor cool, not clear nor soft and that it does not flow in streams nor rise up into waves. And if it isn't a single one of these things what else is it but nothing? Don't tell me."
"Well, well", said the turtle, "If you are determined to think that dry land is nothing, I suppose you must just go on thinking so. But any one who knows what is water and what is land would say you were just a silly fish, for you think that anything you have never known is nothing just because you have never known it."
"And with that the turtle turned away and, leaving the fish behind in its little pond of water, set out on another excursion over the dry land that was nothing. "
It is evident from this significant story that neither can the turtle, who is acquainted with both land and sea, explain to the fish the real nature of land, nor can the fish grasp what is land since it is acquainted only with water. In the same way Arahants who are acquainted with both the mundane and the supramundane cannot explain to a worldling what exactly the supramundane is in mundane terms, nor can a worldling understand the supramundane merely by mundane knowledge.
If Nibbāna is nothingness, then it necessarily must coincide with space (Ākāsa). Both space and Nibbāna are eternal and unchanging. The former is eternal because it is nothing in itself. The latter is spaceless and timeless. With regard to the difference between space and Nibbāna, it may briefly be said that space is not, but Nibbāna is.
The Buddha, speaking of the different planes of existence, makes special reference to a "Realm of Nothingness" (Āki57;yatana).
The fact that Nibbāna is realized as one of the mental objects (vatthudhamma), decidedly proves that it is not a state of nothingness. If it were so, the Buddha would not have described its state in such terms as "Infinite" (Ananta), "Non-conditioned"(Asamkhata), "Incomparable" (Anūpa-meya), "Supreme"(Anuttara), "Highest" (Para), "Beyond" (Pāra), "Highest Refuge" (Parāyana), "Safety" (Tāna), "Security" (Khema), "Happiness" (Siva), "Unique" (Kevala), "Abodeless" (Anālaya), "Imperishable" (Akkhara), "Absolute Purity" (Visuddha), "Supramundane" (Lokuttara), "Immortality" (Amata), "Emancipation" (Mutti), "Peace" (Santi), etc.
In the Udāna and Itivuttaka the Buddha refers to Nibbāna as follows:-
"There is, O Bhikkhus, an unborn (ajāta), unoriginated (abhūta), unmade (akata) and non-conditioned state (asamkhata). If, O Bhikkhus, there were not this unborn, unoriginated, unmade and non-conditioned, an escape for the born, originated, made, and conditioned, would not be, possible here. As there is an unborn, unoriginated, unmade, and non-conditioned state, an escape for the born, originated, made, conditioned is possible. "
The Itivuttaka states:
"The born, become, produced, compounded, made,
And thus not lasting, but of birth and death
An aggregate, a nest of sickness, brittle,
A thing by food supported, come to be, --
'Twere no fit thing to take delight in such.
Th'escape therefrom, the real, beyond the sphere
Of reason, lasting, unborn, unproduced,
The sorrowless, the stainless path that ends
The things of woe, the peace from worries, -- bliss. "
The Nibbāna of Buddhists is, therefore, neither a state of nothingness nor a mere cessation. What it is not, one can definitely say. What precisely it is, one cannot adequately express in conventional terms as it is unique. It is for self-realization (paccattam veditabbo).
Sopādisesa and Anupādisesa Nibbāna Dhātu.
References are frequently made in the books to Nibbāna as Sopādisesa  and Anupādisesa Nibbāna Dhātu.
These in fact are not two kinds of Nibbāna, but the one single Nibbāna receiving its name according to experience of it before and after death.
Nibbāna is attainable in this present life itself if the seeker fits himself for it. Buddhism nowhere states that its ultimate goal can be reached only in a life beyond. Here lies the difference between the Buddhist conception of Nibbāna and the non-Buddhist conception of an eternal heaven which is attainable only after death.
When Nibbāna is realized in the body, it is called Sopādisesa Nibbāna Dhātu. When an Arahant attains Pari-Nibbāna after the dissolution of the body, without any remainder of any physical existence, it is called Anupādisesa Nibbāna Dhātu.
In the Itivuttaka the Buddha savs:
"There are, O Bhikkhus, two elements of Nibbāna. What two? "The element of Nibbāna with the basis (upādi) still remaining and that without basis.
"Herein, O Bhikkhus, a Bhikkhu is an Arahant, one who has destroyed the Defilements, who has lived the life, done what was to be done, laid aside the burden, who has attained his goal, who has destroyed the fetters of existence, who, rightly understanding, is delivered. His five sense-organs still remain, and as he is not devoid of them he undergoes the pleasant and the unpleasant experiences. That destruction of his attachment, hatred and delusion is called the 'Element of Nibbāna with the basis still remaining.'
"What O Bhikkus, is 'the Element of Nibbāna without the basis'?
"Herein, O Bhikkhus, a Bhikkhu is an Arahant ... is delivered. In this very life all his sensations will have no delight for him, they will be cooled. This is called 'the Element of Nibbāna without a basis. "
"These two Nibbāna-states are shown by Him
Who seeth, who is such and unattached.
One state is that in this same life possessed
With base remaining, tho' becoming's stream
Be cut off. While the state without a base
Belongeth to the future, wherein all
Becomings utterly do come to cease.
They who, by knowing this state uncompounded
Have heart's release, by cutting off the stream,
They who have reached the core of dhamma, glad
To end, such have abandoned all becomings. "