金剛經纂要刊定記 3.33

3.33

疏正入定者。此釋經中敷座而坐也。沈掉等者。沈謂昏沈，能引睡眠障定增故。掉謂掉舉，任運攀緣能引散亂亦障定心。又於四儀中，以臥則昏沈，行則掉舉，住則疲倦，唯坐為勝，故不沈掉。然昏沈掉舉，蓋是凡夫，若據如來的無此事，今垂軌則蓋為後人。或曰：「經中但言敷座，焉知入定耶？」故次釋云魏譯等則知入定也。

The commentary's "properly entering samādhi" explains "he spread his seat and sat down" in the sūtra. "Sinking and agitation" and so on: Sinking refers to dullness, which can induce drowsiness and hinder the increase of samādhi. Agitation refers to restlessness; spontaneously grasping at conditions, it can induce distraction and also hinders the mind in samādhi. Furthermore, among the four postures, lying down leads to dullness, walking leads to restlessness, and standing leads to fatigue; only sitting is superior, hence one is free from sinking and agitation. Dullness and restlessness belong solely to ordinary people; as far as the Tathāgata is concerned, he has none of these things, but in bequeathing a standard model now, it is entirely for the sake of posterity. Some may say: "The sūtra only says spreading the seat; how do we know he entered samādhi?" Therefore, the subsequent explanation says that the Wei translation and others make it known that he entered samādhi.