The Pali texts define the seventh fetter as attachment to formlessness. The three worlds are: 1. The world of desire, which is the one we live in, together with hell dwellers, hungry ghosts, animals, asuras, other humans, and the gods of the world of desire; (keep in mind that these are levels of awareness, i.e., humans are more aware than animals, etc.) 2. The world of form, which we visit when our awareness is subtle enough to experience the four jhanas; and 3. The world of formlessness, which we visit when our awareness is subtle enough to experience the immaterial attainments. If we become more crude, through lack of cultivation and pursuit of more sensual pleasures and meanness towards others (militarism, which is institutionalized bullying) ,we can descend into asura consciousness, animal consciousness, and so on to the bottom where we are aware only of despair and unhappiness. The seventh fetter is thus understood as the desire to remain in the world of the four immaterial attainments, the world that is more subtle than the world of the four jhanas. The world of formlessness is usually described as awareness of infinite space, infinite consciousness, nothingness and neither perception nor non-perception. And that nirvana is ineffable, lying beyond the world of formlessness. So the seventh fetter is the desire for the bliss associated with the various stages of concentration on formlessness. The seventh fetter is more subtle than the sixth and it is simply beyond our imagination if we have not yet experienced the mind-bending bliss of the immaterial attainments. Adepts like Ajahn Brahm, who has been there, says the bliss of the four immaterial attainments is far superior to the bliss of the jhanas. We can easily get trapped by words here. We should say that the awareness of the world of formlessness is more subtle than the awareness of the world of form. And the idea of an independent self that experiences the bliss has almost vanished by the time we reach this level of awareness. The awareness just is. It is not possessed by us. See Don’t Take Your Life Personally by Venerable Ajahn Sumedho. And of course we can’t transcend our desire to give up the awareness of infinite space, infinite consciousness, nothingness and neither perception nor non-perception when we haven’t even experienced that level of awareness. But as practitioners, we need to know that if our awareness becomes that subtle, there is still a residual self that needs to let go. Even the seventh fetter, subtle as it is, is still a fetter that keeps us in prison, that denies us the freedom that is inherently within us. No one keeps us locked up but ourselves. We are our own jailors. The Heart of Buddhist Meditation: Satipatthana