Maybe because it’s Winter Solstice, the darkest and hence most spiritual time of year, I’ve been thinking about souls. By soul I mean essence, what makes you uniquely you and me uniquely me.

From one point of view, souls cannot possibly exist. To be human is to shape-shift, shedding one identity after another. Think of all the transitions we go through on the way from infancy to adulthood. In my mid-20s I was a house painter living in Denver. Twenty years later I was a full-time staff writer for Scientific American, a married father of two living in the woods north of New York City. Today I am divorced, and I live and teach at a school in Hoboken, New Jersey.

We undergo micro-changes, too. In the course of a single day I slip through many moods, from glum to giddy. Neuroscientists point out that links between our brain cells are constantly weakening, strengthening, dissolving, re-forming. Constantly! Right now, as you read these words, your brain is re-wiring itself, re-inventing itself. Scientists call this property neuroplasticity.

Read the rest of this article in Scientific American