首楞嚴義疏注經 6.382

6.382

《涅槃》第四，迦葉問云：「云何如來先許比丘食三淨肉？」佛言：「隨事漸制故耳。」復有七種、九種，今言五者，隨經增減，以意配數。佛以方便權許令食，非究竟說。

In the fourth [fascicle] of the Nirvāṇa [Sūtra], Kāśyapa asks: "Why did the Tathāgata previously allow monks to eat the three kinds of pure meat?" The Buddha said: "It was merely because I formulated the precepts gradually in accordance with circumstances." There are also seven kinds and nine kinds; here speaking of five, it increases or decreases according to the sūtra, matching the number as appropriate. The Buddha used skillful means to provisionally allow eating it; it was not a definitive teaching.