Here is a variant of Wittgenstein’s diary thought experiment, using the inner light in place of private, inner sensation. Suppose I decide to try to discover whether and when I suffer from zombie episodes, episodes of phenomenal absence. So I keep a diary, and I write ‘L’ in the diary on those days when my inner light is switched on. Later, I tell myself, I will be able to look back through the diary and, seeing an ‘L’, be sure that I was phenomenally present on that day, that all was not dark inside. Well, how could I trust any previous occurrences of ‘L’ marked in my diary, even if (contra Wittgenstein) I were capable of unilaterally identifying when my private, inner light was on? After all, if I was phenomenally absent on that day—away with the zombies, so to speak—I would have written an ‘L’ in my diary even in my zombie state. So the ‘L’ can tell me nothing. It seems to have no use even in a private language.

–Embodiment and the inner life: Cognition and Consciousness in the Space of Possible Minds by Murray Shanahan