Sri Aurobindo
Letters of Sri Aurobindo
Volume III - Part 4
Once the being or its different parts begin to ascend to the planes above, any part of the being may do it, frontal or other. The sanskara that one cannot come back must be got rid of. One can have the experience of Nirvana at the summit of the mind or anywhere in those planes that are now superconscient to the mind; the mind spiritualised by the ascent into Self has the sense of laya, dissolution of itself, its thoughts, movements, sanskaras into a superconscient Silence and Infinity which it is 
unable to grasp,– the Unknowable. But this would bring or lead to some form of Nirvana only if one makes Nirvana the goal, if one is tied to the mind and accepts its dissolution into the Infinite as one’s own dissolution or if one has not the capacity to reorganise experience on a higher than the mental plane. But otherwise what was superconscient becomes conscient, one begins to possess or else be the instrument of the dynamis of the higher planes and there is a movement, not of liberation into Nirvana, but of liberation + transformation. However high one goes, one can always return, unless one has the will not to do so.