lit. 'That which cannot or should not be thought, the unthinkable, ...
lit. 'That which cannot or should not be thought, the unthinkable, incomprehensible, impenetrable, that which transcends the limits of thinking and over which therefore one should not ponder.
These 4 unthinkables are:
the sphere of a Buddha (buddha-visaya),
of the meditative absorptions (jha^na-visaya),
of karma-result (kamma-vipa^ka), and
brooding over the world (loka-cinta^), especially over an absolute first beginning of it
"Therefore, o monks, do not brood over the world as to whether it is eternal or temporal, limited or endless .... Such brooding, O monks, is senseless, has nothing to do with genuine pure conduct (s. a^dibrahmacariyaka-si^la), does not lead to aversion, detachment, extinction, nor to peace, to full comprehension, enlightenment and Nibba^na, etc." (S.56.41).