法華義疏 2.61

2.61

One who adopts this posture is unlike worldly people and also differs from non-Buddhists. Non-Buddhists and the like constantly stand or raise one foot; because the body is not upright, the mind is not correct. Sitting cross-legged is the constant posture of all buddhas; one who adopts this posture has an upright body and a correct mind. Intending to quiet the mind, one first makes the body upright. Furthermore, adopting this posture makes it easy to realize the path, and Māra gives rise to fear; hence one sits cross-legged.