One morning after her accident, a woman I'll call Kate awoke in a daze. She looked at the man next to her in bed. He resembled her husband, with the same coppery beard and freckles dusted across his shoulders. But this man was definitely not her husband.

Panicked, she packed a small bag and headed to her psychiatrist's office. On the bus, there was a man she had been encountering with increasing frequency over the past several weeks. The man was clever, he was a spy. He always appeared in a different form: one day as a little girl in a sundress, another time as a bike courier who smirked at her knowingly. She explained these bizarre developments to her doctor, who was quickly becoming one of the last voices in this world she could trust. But as he spoke, her stomach sank with a dreaded realisation: this man, too, was an impostor.

Kate has Capgras syndrome, the unshakeable belief that someone - often a loved one, sometimes oneself - has been replaced with an exact replica. She also has Fregoli syndrome, the delusion that the same person is taking on a variety of shapes, like an actor donning an expert disguise. Capgras and Fregoli delusions offer hints about an extraordinary cognitive mechanism active in the healthy mind, a mechanism so exquisitely tuned that we are hardly ever aware of it. This mechanism ascribes to each person a unique identity, and then meticulously tracks and updates it. This mechanism is crucial to virtually every human interaction, from navigating a party to navigating a marriage. Without it, we quickly fall apart. ...[Continue reading on Aeon Magazine]