"On the night of November 10, 1619, René Descartes dreamed that he was stumbling down the street pursued by ghosts."

"His right side was weak, and a whirlwind spun him violently on his left foot; he limped past a man whom he suddenly realized he knew, then turned to speak to a different man, who told him to go see Monsieur N., who had something to give him. Descartes knew what it was: a melon. A lesser thinker might have seen in this dream a craving for cantaloupe. But, to Descartes, its vividness seemed to suggest a clear disjunction between the body and the mind: in dreams the body lies dormant while the mind runs free...."

Writes Amanda Gefter, in "What Are Dreams For? Converging lines of research suggest that we might be misunderstanding something we do every night of our lives" (The New Yorker).

[T]he American sleep scientist William Dement and the French neuroscientist Michel Jouvet each observed that, when cats enter REM, they lose all muscle tone. The same is true for humans—the result, Jouvet discovered, of inhibitory signals, sent by the brain to the spinal cord, that paralyze the body. 

When this paralysis fails, it results in REM behavior disorder, in which people may talk, kick, and even act out violently in their sleep. When it persists, we experience “sleep paralysis,” in which we wake up unable to move. When the system works as it should, Jouvet wrote, we enjoy “paradoxical sleep”: our brains come alive with wild visions as our bodies go motionless between the sheets.... 

... Descartes’s difficulty walking could well reflect the fact that his real-life legs are paralyzed. He leans to the left—which, by all accounts, is the side on which he is sleeping.