Chih-I portrays the Ultimate Truth by equating it with empty space in a house. Empty space in a house has neither roof beams nor pillars. The substance of a house, empty space, thus represents the Ultimate Truth. On the other hand, the roof beams and pillars are taken to analogize the cause and effect of Buddhahood. This is because if a house has no void, it cannot contain and receive anything. If the cause and effect of Buddhahood are not based on the Ultimate Truth as substance, they cannot sustain themselves. Thus, Chih-i holds that it is necessary to single out the correct substance that consists of only one empty space, upon which everything is able to function. (Vol. 2, Page 407-408)