Buddhism points out that although Nirvana is, there is no self to perceive it. As Buddhism denies a permanent self, the question of what Nirvana is experimentally does not arise. Nirvana is not a state of mind which is to be produced but is what is realized when the long-cherished notion of "I" is given up. Nirvana, in short, is the miracle of egoless being. The Buddha's doctrine of the soul was stated in negative terms because he was controverting current misconceptions. He explained this in Alagadupama Majjhima, 1, 135: "Even in this present life, my brethren, I say that the soul is indefinable. Though I say and teach thus, there are those who accuse me falsely of being a nihilist, of teaching the non-existence and annihilation of the soul. That is what I am not and do not teach."
-- Notebooks Category 22: Inspiration and the Overself > Chapter 3: The Overself's Presence > # 406