"Causation" in this passage refers to "moral causation." The Buddhist concept of karma acknowledges that [volitional] good/bad deeds, thoughts, and so forth result in good/bad effects. Thus the import of the question posed by the "fox" is whether or not the enlightened person is subject to karma. Hyakuj?'s answer, in effect, affirms that the enlightened person is subject to moral causation. ...
D?gen's employment of this story in the "Daishugy?" chapter of the Sh?b?genz? implies that, on one level, he thinks Hyakuj?'s answer indeed provides a "remedy" for the old man's predicament. Yet D?gen was rarely content with merely citing traditional Zen interpretations of passages; typically, he sought to push his students to a further understanding by a creative reinterpretation of a passage. Lest his disciple therefore think this not-ignoring/recognition of causation is de facto a release from it in an ultimate sense, D?gen answers that the passage means "cause and effect are immovable." In other words, moral causation, for D?gen, is an inexorable fact of human existence.
Given this fact, Ej? then asks how we can ever "escape" moral causation. D?gen's response is enigmatic: "Cause and effect arise at the same time." Nowhere in the Sh?b?genz? Zuimonki does he further clarify this passage. However, the key to understanding this statement can be gleaned from his discussion of causation in the "Shoakumakusa" chapter of the Sh?b?genz?, wherein he observes that "cause is not before and effect is not after." As Hee-Jin Kim explains, D?gen saw cause and effect as absolutely discontinuous moments that, in any given action, arise simultaneously from "thusness." Therefore,
... no sooner does one choose and act according to a particular course of action than are the results thereof (heavens, hells, or otherwise) realized in it .... Man lives in the midst of causation from which he cannot escape even for a moment; nevertheless, he can live from moment to moment in such a way that these moments are the fulfilled moments of moral and spiritual freedom and purity in thusness.