Chances are that you may have experienced it fleetingly. Studies carried out with college students have found that brief episodes are common in young people, with a prevalence ranging from 30 to 70 percent. It can happen when you’re jet-lagged, hungover, or stressed. But for roughly 1 to 2 percent of the population, it becomes persistent, and distressing. Dr. Elaine Hunter, a clinical psychologist working at the Depersonalization Disorder Clinic in London, U.K., says that it’s frequently misdiagnosed. “People often think it is just to do with depression,” she tells me. While depersonalization can be a symptom of anxiety and depression, it can also occur on its own.

Research suggests that areas of the brain that are key to emotional and physical sensations, such as the amygdala and the insula, appear to be less responsive in chronic depersonalization sufferers. You might become less empathetic; your pain threshold might increase. These numbing effects mean that it’s commonly conceived as a defense mechanism; Hunter calls it a “psychological trip switch” which can be triggered in times of stress.

Dr. Elena Bezzubova, a Russian psychoanalyst who treats people with depersonalization in California, calls it a painful absence of feeling. “A mother comes to me and says, ‘My son is in prison, I received a letter from him. I do not care, but it bothers me. Please prescribe me something to cry.’”

It might be the implications of the numbing, as opposed to the actual numbing itself, that cause the most distress. Have you ever played that game when you repeat a word over and over again until it loses all meaning? It’s called semantic satiation. Like words, can a sense of self be broken down into arbitrary, socially-constructed components?

That question may be why the phenomenon has attracted a lot of interest from philosophers. In a sense, the experience presupposes certain notions of how the self is meant to feel. We think of a self as an essential thing—a soul or an ego that everyone has and is aware of—but scientists and philosophers have been telling us for a while now that the self isn’t quite as it seems. Psychologist Dr. Bruce Hood writes in The Self Illusion that there is no center in the brain where the self is generated. “What we experience is a powerful depiction generated by our brains for our benefit,” he writes. Brains make sense of data that would otherwise be overwhelming. “Experiences are fragmented episodes unless they are woven together in a meaningful narrative,” he writes, with the self being the story that “pulls it all together.” In The Ego Trick, Julian Baggini writes that people are, as the 18th century philosopher David Hume wrote in A Treatise of Human Nature, “bundles of different perceptions.” “The unity [of self that] we experience, which allows us legitimately to talk of ‘I,’ is a result of the Ego Trick—the remarkable way in which a complicated bundle of mental events, made possible by the brain, creates a singular self, without there being a singular thing underlying it,” Baggini writes.