俱舍論疏 24.109

24.109

論「空處二十八至互為因故」，釋上三地數漸減也。無色不緣下地，故無法智，除八行也。不緣四靜慮滅，又餘十六行，總除二十四行。由此唯有二十八。識處更減四行，二十四。無所有處更減識處四，所以唯二十。

The Śāstra says: "In the sphere of infinite space there are twenty-eight […] because they serve as mutual causes." This explains how the numbers gradually decrease in the upper three stages. The formless realm does not take lower stages as objects; hence there are no dharma knowledges, removing eight aspects. Not taking the cessation of the four dhyānas as objects removes another sixteen aspects, so twenty-four aspects in total are eliminated. For this reason, there are only twenty-eight. In the sphere of infinite consciousness, four more aspects are subtracted, leaving twenty-four. In the sphere of nothingness, the four of the sphere of consciousness are further subtracted; hence there are only twenty.