Well the idea of nirvana is freedom from everything. Everything, included preconceived notions and thoughts, which means if you think that you are in nirvana, you have not freed yourself, therefore you arent in nirvana.

At least thats how I have always interpretted it.

I dont know if Id ever be able to acheive it, because every time I feel that light wispy touch of enlightenment and revelation, I try to grasp it, because its such a wonderful, unique, one of a kind feeling that comes along so rarely.