The Bhagavat [Blessed One = the Buddha] said, "Noble son, how have you previously trained/ disciplined your mind?"
Subhadra replied, "Bhagavat, I reflected intensely upon the fact that the [Realm of ] Desire is impermanent, unpleasant, and utterly impure, and realised that the [Realm of] Form is permanent, pleasant, and utterly pure. Having realised thus, I severed the kleshas of the Realm of Desire and attained the sphere [ayatana] of Form. In that way, I previously trained / disciplined my mind.
"Then, when I investigated the [Realm of] Form, I realised that form is impermanent and is like a sore, an ulcer, poison, or a thorn. I saw that the [Realm of] Formlessness is permanent, pure and peaceful. Having realised this, I severed the kleshas of the Realm of Form and attained the sphere of Formlessness. In that way, I previously trained / disciplined my mind.
"Then, when I investigated ideation [samjna = the making of thoughts and ideas in the mind], I realised that it is impermanent and is like a sore, an ulcer, poison, or a thorn, and I attained the samadhi of the sphere of neither-ideation-nor-non-ideation [naivasa-njaanasa-njaa]. [I realised that] the sphere of neither-ideation-nor-non-ideation is all-knowing awareness [sarvajna-jnana], peaceful, pure, irreversible and unchanging. In that way, I previously trained / disciplined my mind."
The Bhagavat said, "Noble son, how have you trained / disciplined your mind? What you have attained is the samadhi of the sphere of neither-ideation-nor-non-ideation, but that is still ideation. If Nirvana is devoid of ideation, why do you term this 'Nirvana'? Noble son, if you previously disdained coarse ideation, why are you attached to subtle ideation, not knowing that it is inferior? Even that sphere of neither-ideation-nor-non-ideation may be termed 'ideation'. It too is like a sore, an ulcer, poison, or a thorn. Noble son, though your teacher, Udraka-Ramaputra, has acute faculties and is prudent, he worships the sphere of neither-ideation-nor-non-ideation. If he still is embodied in a low-grade body, what need is there to say anything further!"