On Certainty #383: The argument "I may be dreaming" is senseless for this reason: if I am dreaming, this remark is being dreamed as well and indeed it is also being dreamed that these words have any meaning.
I beg to differ. It is a plain fact that people have dreams in which they know that they are dreaming, and in which they think to themselves, 'I am now dreaming.' In those dreams they are not dreaming that they are dreaming, if dreaming that p entails that one does not know that p. For they they do know that they are dreaming despite their being asleep and dreaming.
And so Ludwig Wittgenstein in the above passage gives us no reason to dismiss the Cartesian dream argument for skepticism about the external world.
I once had an extremely vivid dream about my dead cat, Maya. There she was, as (apparently) real as can be. I saw her, I touched and petted her, I heard her. It was all astonishingly vivid and coherent. There was an ongoing perceiving in which visual, tactile, and auditory data were well-integrated. And yet I knew within the dream that she was dead, and I knew that I had buried her in April 2001 in the desert behind the house, and that coyotes had dug her up and had consumed her rotting corpse.
And so I began to philosophize within the dream: I know that Maya is dead and that I am dreaming, and so these perceptions, as vivid and coherent as they are, cannot be veridical. Coherence is no guarantee of veridicality. Neither is their "force and vivacity" to borrow a phrase from Hume. I did not dream that I was dreaming, I knew that I was dreaming; and I did not dream the reasoning in the third-to-last sentence, I validly executed that reasoning. And the meanings of the terms in the reasoning were in no way affected by their being grasped within a dream.
Wittgenstein seems to be assuming that, for any proposition p, if one becomes aware that p while dreaming, then one has dreamt that p in a sense that entails that one does not know that p. But this assumption is false, as Descartes appreciated. Becoming aware that 2 + 3 = 5 while dreaming is consistent with knowing its truth in the way that dreaming that one is sitting before a fire, when one is lying in bed, is not consistent with knowing its truth.
So there is no reason to deny that one can become aware that one is dreaming while dreaming. To become aware that one is dreaming while dreaming is not to dream that one is dreaming in a sense that implies that one is not in reality dreaming. And to use words within a dream is not to dream the meanings of those words in a sense that implies that they do not in reality have those meanings.
My point, again, is that Wittgenstein in the above passage gives us no reason to dismiss the Cartesian dream argument for skepticism about the external world.