Generally treated as an absolute (ungradable) adjective; comparative and superlative forms are rare and considered non-standard.
It is inconceivable that he would betray his friends.
Немыслимо, чтобы он предал своих друзей.
The idea of living on Mars was once inconceivable.
Идея жизни на Марсе когда-то была невообразимой.
The complexity of the universe is inconceivable.
Сложность вселенной непостижима.