俱舍論疏 24.158

24.158

論「近分根本至必入根本」，釋所以也。下三地、未至與根本地受不同故，轉入異受少艱難故。若欣上心強即入根本，若欣劣者即不能入。第四靜慮及四無色未至、根本同一捨受，離下地染必欣上故，所以上地定入根本。

On the Śāstra's statement: "Preparatory stages and fundamental stages up to definitely enters the fundamental": This explains the reason. Because the feeling (vedanā) of the threshold and the fundamental stage of the lower three grounds differs, transitioning into a different feeling is somewhat difficult. If the mind aspiring to the higher is strong, one enters the fundamental; if one's aspiration is weak, one cannot enter. The threshold and fundamental stages of the fourth dhyāna and the four formless absorptions share the same feeling of equanimity (upekṣā); abandoning the defilement of a lower ground necessarily involves aspiring to the higher; therefore, the higher grounds definitely enter the fundamental.