June 11 (1826).—Bad dreams about poor Charlotte. Woke, thinking my old and inseparable friend beside me; and it was only when I was fully awake that I could persuade myself that she was dark, low, and distant, and that my bed was widowed. I believe the phenomena of dreaming are in a great measure occasioned by the double touch, which takes place when one hand is crossed in sleep upon another. Each gives and receives the impression of touch to and from the other, and this complicated sensation our sleeping fancy ascribes to the agency of another being, when it is in fact produced by our own limbs acting on each other. Well, here goes—incumbite remis.
From Scott's Journal.
Walter Scott seems to have a somewhat practical conception of the cause of "double touch", as related to dreaming. As a comparison, Jennifer Ford in her "Coleridge on Dreaming: Romanticism, dreams, and the medical imagination" notes:
...Coleridge was not the first person to explore the possibilities of single and double sense phenomena, but his investigations are particularly coloured by his profound and often complex meditations on dreaming and dreams...Single and double touch seem to be inexorably connected for him with organs and the flesh, but most specifically with experiences of sexuality and the sexual organs. It is not only touch which can be delineated into single and double manifestations: vision also has this capacity, and it is quite possible that both vision and touch are at work in the derangement of the circulation in nightmairs.
The first mention Coleridge makes of single and double sense awareness occurs in September (25th) 1798..."Dined at the Table-d'hot/Wine Soup with currants in it...That night sate up till 4 in the morning and versified 200 lines/went to bed, could not sleep - saw curious instance of single and double vision...