One of the deepest mysteries of the human mind is that it doesn't feel like part of the body. Our consciousness seems to exist in an immaterial realm, distinct from the meat on our bones. We feel like the ghost, not like the machine.
This ancient paradox—it's known as the mind-body problem—has long perplexed philosophers. It has also interested neuroscientists, who have traditionally argued that the three pounds of our brain are a sufficient explanation for the so-called soul. There is no mystery, just anatomy.
In recent years, however, a spate of research has put an interesting twist on this old conundrum. The problem is even more bewildering than we thought, for it's not just the coiled cortex that gives rise to the mind—it's the entire body. As the neuroscientist Antonio Damasio writes, "The mind is embodied, not just embrained."
The latest evidence comes from a new study of probiotic bacteria, the microorganisms typically found in yogurt and dairy products. While most investigations of probiotics have focused on their gastrointestinal benefits—the bacteria reduce the symptoms of diarrhea and irritable bowel syndrome—this new research explored the effect of probiotics on the brain.