Ākiṃcanyāyatana ("Sphere of Nothingness"): In this sphere formless beings dwell contemplating upon the thought that "there is no thing".
This is considered a form of perception, though a very subtle one.
This was the sphere reached by Ārāḍa Kālāma (Pāli:
Āḷāra Kālāma), the first of the Buddha's two teachers; he considered it to be equivalent to enlightenment.
etymology: Ākiṃcanyāyatana or Ākiñcaññāyatana (Tib: ci yang med) (literally "lacking anything")