user-1: In the book, What the Buddha Taught, Walpola Rahula writes on page 65, "It is the vague feeling 'I AM' that creates the idea of self which has no corresponding reality, and to see this truth is to realize Nirvana, which is not very easy." 
I understand this to mean that 'Nirvana' is "seeing a particular truth". Therefore it seems reasonable to conclude that not seeing this truth is to not achieve Nirvana, and that losing sight of this truth is to lose Nirvana. 
So, is Nirvana an attitude or capacity or insight that can be lost, or is it a solid destination or eternal achievement as some claim/hope?

user295: Completely in agreement with @santa100.
If I am to add to it, it is like a singularity where all conventional laws break down. So what happens beyond on in this state cannot be communicated through conventional wisdom or language. 
Nirvana is the experience beyond the singularity. Once you experience it you have experienced it. You cannot unexperience it. Say you have gone and see Rome. This is a fact. Is it possible to turn back time so you have not seen Rome. Not possible. Likewise once you have seen what is beyond the singularity you cannot be made to be brought to the state where you have not seen it. 
Also you experience Nirvana when you are in Pala Samapathi. When you come out of it you don't experience it but the fact you have seen it will not change. Since you know how you got there you can get to it when you want. This experience has a lasting impression / imprint on you. So your ways will change once you come out of the Samapathi. 
To further elaborate. The lasting impression is like once your where in Rome had a life changing experience. This sticks. 

user1003: It'll be very difficult to define Nibbana within our mundane scope of dualistic concepts of temporal/eternal. That's why the suttas refer to Nibbana as an "apophasis" (the use of negation language to express a transcendental supramundane state that is ineffable that we can only 'touch' by personal experience truth or reality, such as awakening or nibbana). An example from Samyutta 43 where the Buddha said: "There is, monks, that base where there is neither earth, nor water, nor heat, nor air; neither the base of the infinity of space, nor the base of the infinity of consciousness, nor the base of nothingness, nor the base of neither-perception-nor-non-perception; neither this world nor another world; neither sun nor moon. Here, monks, I say there is no coming, no going, no standing still; no passing away and no being reborn. It is not established, not moving, without support. Just this is the end of suffering."

-1: <p>In the book, <em>What the Buddha Taught</em>, Walpola Rahula writes on page 65, <em>"It is the vague feeling 'I AM' that creates the idea of self which has no corresponding reality, and to see this truth is to realize Nirvana, which is not very easy."</em>  </p>

<p>I understand this to mean that 'Nirvana' is "seeing a particular truth". Therefore it seems reasonable to conclude that not seeing this truth is to not achieve Nirvana, and that losing sight of this truth is to lose Nirvana. </p>

<p>So, is Nirvana an attitude or capacity or insight that can be lost, or is it a solid destination or eternal achievement as some claim/hope?</p>
 295: <p>Completely in agreement with @santa100.</p>

<p>If I am to add to it, it is like a singularity where all conventional laws break down. So what happens beyond on in this state cannot be communicated through conventional wisdom or language. </p>

<p>Nirvana is the experience beyond the singularity. Once you experience it you have experienced it. You cannot unexperience it. Say you have gone and see Rome. This is a fact. Is it possible to turn back time so you have not seen Rome. Not possible. Likewise once you have seen what is beyond the singularity you cannot be made to be brought to the state where you have not seen it. </p>

<p>Also you experience Nirvana when you are in Pala Samapathi. When you come out of it you don't experience it but the fact you have seen it will not change. Since you know how you got there you can get to it when you want. This experience has a lasting impression / imprint on you. So your ways will change once you come out of the Samapathi. </p>

<p>To further elaborate. The lasting impression is like once your where in Rome had a life changing experience. This sticks. </p>
 1003: <p>It'll be very difficult to define Nibbana within our mundane scope of dualistic concepts of temporal/eternal. That's why the suttas refer to Nibbana as an "apophasis" (the use of negation language to express a transcendental supramundane state that is ineffable that we can only 'touch' by personal experience truth or reality, such as awakening or nibbana). An example from Samyutta 43 where the Buddha said: "There is, monks, that base where there is neither earth, nor water, nor heat, nor air; neither the base of the infinity of space, nor the base of the infinity of consciousness, nor the base of nothingness, nor the base of neither-perception-nor-non-perception; neither this world nor another world; neither sun nor moon. Here, monks, I say there is no coming, no going, no standing still; no passing away and no being reborn. It is not established, not moving, without support. Just this is the end of suffering."</p>
