大般涅槃經義記 3.294

3.294

第二句中，始聞不受名「不信醫」，「若在皮」下顯不信相，現身名皮，未來世中證性之身深遠如筋，性在現身名在皮裹。現在身中膿血不淨，何緣不出？性在未來深遠身中名在筋裏，當有現無，不應可見。迷理疑教，故言「云何欺誑於我」。

In the second clause, hearing for the first time and not accepting is called "not believing the physician"; from "if it is in the skin" down, the marks of disbelief are revealed: the present body is called the skin, while the body that realizes the nature in a future life is deep and distant like sinew; the nature being in the present body is called being inside the skin. In the present body, pus and blood are impure; for what reason does it not emerge? The nature being in the deep and distant body of the future is called being within the sinew; being something that will exist in the future but is absent at present, it should not be visible. Being deluded about the principle and doubting the teaching, he therefore says, "why do you deceive me?"