One day in the late 19th century, the Austrian physicist and philosopher Ernst Mach gets on a bus. As he stares down the aisle, he sees a person at the other end, a character he dismisses as a ‘shabby pedagogue’. In the next instant, Mach realises the shabby pedagogue is none other than himself, staring out from a mirror positioned at the back of the bus.

For a few moments, Mach had become a stranger to himself. Psychologists estimate that around three-quarters of us will experience similar symptoms of self-detachment at some point in our lives. If you’ve been through trauma, or narrowly escaped a nasty accident, you might recall how a sense of unreality can wash over you, how you suddenly disconnect from yourself, or feel as if you’re floating in the air and watching from above. These states of mind seem to function as an experiential airbag, allowing us to deal with life-threatening dangers which would otherwise be overwhelming.

Luckily, with care and patience, the airbag can usually be wrapped up after the traumatic event, and we find ourselves back in our bodies and our lives. But in some unlucky cases, the protective mechanism gets ‘stuck’. People can be trapped outside themselves, unable to inhabit their own experiences, feelings and thoughts – like Mach, if he were unable to reconnect to himself after spying the shabby pedagogue in the mirror.

This is now Jane Charlton’s experience of her day-to-day life. I met Jane, a British woman in her mid-30s, about a year ago, when she gave a moving talk to a packed audience at an interdisciplinary workshop I’d organised in London. It is one thing to study a phenomenon in the lab, or from a philosopher’s armchair perspective. But it’s quite another to meet someone face-to-face who is living with the condition that you’re using to ground this or that theory or interpretation.

If I quieten my mind, I can almost taste the colour and richness of life as I knew it before, says Jane. It comes with a sense of expectation, a feeling of being an agent in changing and plotting a course through the world. This is, I think, the very act of ‘living’, which I bear witness to in others, all day, every day. I still understand it academically, but I can barely remember what it feels like. These days I’m in a constant state of grief; I feel as if I’m grieving for my own death, even if I seem to be around to witness it.

Today, Jane and I are sitting in a quiet café on Lamb’s Conduit Street in Bloomsbury, a road that inspired parts of Virginia Woolf’s novel Jacob’s Room (1922). Jane works as the head of European Human Rights at the UK Ministry of Justice. We talk about cats, Portuguese wines and philosophy. She laughs, makes jokes, tells me she likes spending time in France and speaking French. From where I sit – an ‘outside’ observer – she behaves like any other Londoner, enjoying a conversation and a cup of coffee on a cold and rainy afternoon. Yet on the ‘inside’, Jane feels as if she doesn’t fully exist. To herself, she is unreal.

Jane suffers from depersonalisation disorder (DPD) – a condition that typically manifests as a profound and distressing feeling of estrangement from one’s own self and body, including one’s experiences, memories and thoughts. Often, depersonalisation is accompanied by derealisation, an alienation from one’s surroundings and environment. Sufferers report feeling like zombies, robots or machines, just going through the motions of their own lives. This disorder has been recognised in some form since the late 19th century, and according to studies in the US and UK is estimated to affect between 1 to 2 per cent of the population. But it remains poorly understood.

Just as a transparent window might become visible only when it cracks, I became interested in what DPD might reveal about certain understudied aspects of our experience of selfhood – and, ultimately, consciousness itself. In recent years, it’s become fashionable for philosophers to question the idea that the ‘self’ is real, or to suggest that it’s little more than an ‘illusion’ that our brain creates in order to keep us alive in a constantly changing world. Yet if the self is a mere sham or a trick, why does the loss of ‘self-illusion’ trigger such dramatic feelings of unreality? Why does losing a link to your self make you feel as if you are dead or sleepwalking? If depersonalisation is a misfiring of some psychological coping mechanism, why is living with the condition so unbearable?

It was 29 March 2002. I was 18, visiting my boyfriend, Marcel, at his family’s house in France. (I’ve changed his name to protect his privacy.) We were there alone. The days were unseasonably warm, and quiet. We usually got up late, and drove to the supermarket in the afternoons to buy beer and crisps, in case any of his friends were coming around later that night to play music.

That night, they drank and smoked weed and played guitars as usual. Marcel had some cannabis in the form of resin. At some point he started to heat it in a teaspoon with a lighter, and then mixed it with some yoghurt in a tub. I ate one. It didn’t seem to be doing anything, so I ate another an hour later.

I’d smoked cannabis once before, on a hot summer day after I finished my A-levels. I didn’t like the effect: it made the world feel muffled, and me a bit divorced from it, but it passed quickly. This time, after eating the second yoghurt, something terrifying happened. My perception drew back into my head, almost as though I was now looking at the world from the back of my own eye sockets. I perceived a delay between an external event, and my brain understanding or processing it. Suddenly there was a fracture between the world and me. While my body was still in the world, my mind had become a disengaged observer.

According to the DSM-IV, the ‘standard’ omnibus of mental disorders, DPD tends to occur alongside symptoms of anxiety, panic and depression. Aside from drug use, other triggers include severe stress, abuse and trauma. In part because of these correspondences, some researchers have suggested that depersonalisation might not be a disorder of its own. However, there are good reasons to see DPD as a distinct syndrome. Empirical studies suggest that anxiety is strongly associated with DPD only in cases where the latter condition is mild. By contrast, people with severe DPD show no such correlation – a counterintuitive finding, since one would expect anxiety to be more acute the higher the depersonalisation becomes. Yet it might make sense if you see DPD as affecting the special connection you have with your embodied self, a connection that might in turn be necessary to experience various forms of anxiety. This interpretation is consistent with a 2014 finding that patients with clinical DPD had a reduced ability to detect their internal bodily states, such as their own heartbeat, and to display empathy.

People with DPD are aware that their feelings are a subjective phenomenon, not an objective reality

Similarly, a study from 2016 indicated that people with high levels of depersonalisation were less inclined to display self-bias in ‘mirroring’, where you automatically imitate people around you. People with low levels of depersonalisation were quicker to mirror actions related to themselves (such as seeing their own face being touched) than when actions related to others (such as witnessing someone else being touched on their face). But people with high levels of depersonalisation showed no such tendency: seeing their own face didn’t facilitate mirroring. Once again, identification with the self appears to have been disrupted.

Significantly, unlike other disorders of selfhood, such as schizophrenia and psychosis, Jane and people with DPD are aware that their feelings of disconnection are a subjective phenomenon rather than an objective reality. In the technical language of psychology, their ‘reality-testing’ remains intact – they still appreciate the difference between their own perception of a situation and the possibility that its true nature might be quite different.

During that first episode in France, hours followed where I sought reassurance from those around me, wanting to touch and talk to them constantly. I wanted to check that I still existed. Eventually, exhausted, I slept, in the hope that it would pass overnight.

It didn’t. The next morning, the shift in perception remained, and would in fact remain for every second of every day for the next three years. We saw on the news that the Queen Mother had died. I felt sick with the normality of all the reporting, and the ceremonial preparations. Over the course of one evening, I had disintegrated, but everything around me was continuing. It was as though all the constituent parts of me were still working, but an essential and vital element of my self, of my person, was missing.

I’m conscious even as I say this that I must have a functioning inner life; one that is capable of articulating this experience. But the capacity to knit that awareness into a narrative that I can occupy and own is missing.

Just what is this bridge between ‘I’ and ‘myself’ that Jane has lost? It evokes an important distinction proposed by the German philosopher Edmund Husserl, one of the founding figures of the philosophical school known as phenomenology. Husserl believed that the body has a dual aspect: it was both a physical, organic object in the world that could be perceived as such from the outside (Körper in German) – but the body was also lived and experienced from the inside, within an irreducible, first-person perspective (Leib). An anorexic person, for example, might experience her body subjectively as being too fat, whereas an external observer would see her as being extremely thin, just skin and bones. Crucially, for most people, the Leib as it is experienced from the inside feels more real than the Körper as it is observed from without. Yet for those with DPD, somehow the Körper has taken precedence.

A friend of mine, Sarah, who also has depersonalisation, describes it like this. Imagine you’re holding a snowglobe in your hands. The world is in the globe, but you feel like you’re on the outside of the glass. Somehow, you are unable to connect the ‘I’ looking in with the world in the globe. That connection and its absence makes all the difference to whether or not I feel I am alive.

I’ve now had four major episodes of depersonalisation disorder. I am currently five years into an episode that doesn’t show any sign of abating. The experience changes the more chronic it becomes. Early on, the sensation was peculiarly physical. Right after eating the yoghurt, I had a clear memory of how I had felt a short while earlier, and could identify the distinct, physical changes that were impeding me from feeling well. My vision was not tracking objects properly around the room; I felt dizzy; my reactions were delayed. These days, I experience depersonalisation as mostly a lack of narrative. Moments seem to melt away as soon as they have passed, and life goes by as a series of unrelated frames.

Jane’s loss of narrative tracks another set of phenomenological concepts, touching on processes rather than bodies. For Husserl, subjective experience has an intrinsic temporal dimension – events flow from the past through the present towards the future. To be a subject, he says, is to ‘live through’ something (Erleben). This dimension seems to be lacking in DPD, even though the sufferers retain a contrasting capacity: to survive or stay alive, as a merely organic creature (Leben).

If we get stuck in I-as-object, we no longer feel we’re affecting and being affected by the world and others

These concepts from phenomenology can help us reframe some of the existing debates within philosophy of mind. For example, philosophers such as Husserl, Jean-Paul Sartre, Dan Zahavi and Dorothée Legrand have written about the notion of pre-reflective self-consciousness: a kind of primitive, bedrock sense of oneself. Pre-reflective self-consciousness corresponds to the taken-for-granted awareness that this thing we know as ‘I’ is the subject of its own experience, as opposed to the object of our own or another person’s. For example, when I introspect about my own thoughts, survey myself in the mirror, or read my vital signs off a smartwatch, I take myself as an object of experience. But when I feel the wind in my hair, immerse myself in the pleasure of reading a novel, or suffer from toothache, I feel as if I am a subject.

Perhaps the feeling of being fully and properly alive, then, depends on a kind of pre-reflective selfhood that allows us to move seamlessly between the objective and subjective points of view. In phenomenological terms, we might say that we shift back and forth across the bridge between the external realm of Körper/Leben and the internal one of Leib/Erleben. When it is functioning well, this experiential openness remains tacit and imperceptible, even as it pervades and sustains almost everything we do. But when we lose the link, as Jane seems to have lost it, the feeling of inhabiting reality itself also slips away. We get stuck in the I-as-object realm, and no longer have a sense of affecting and being affected by the world and others.

When the depersonalisation is very deep, I still seek to ‘be’ and be with someone else, because it feels like that constant source of interaction is the only thing that allows me to maintain a connection with the world. This includes seeking physical contact with whoever I’m with. It’s like I need to be that other person because my own sense of self is not strong enough to sustain me.

As the years have passed and I’ve been in and out of bouts of depersonalisation, I’ve built up a body of evidence to suggest that I will never lose touch with myself completely, though I still feel uncomfortably close to losing that tenuous link. Even when I’m struggling to knit one second together with the next, my theoretical understanding of who I am has remained. This lack of a slide into psychosis is one of the defining features of the illness.

The experience of living with DPD indicates that a ‘theoretical’, objective grasp of who we are isn’t sufficient for establishing a true sense of oneself. But is this sense a real and important feature of the world, or just one of those peculiar phenomena that philosophers invent to make life even more difficult than it already is? I start to ask Jane what she thinks, but she gently interrupts me and gets up without finishing her tea. I just need to check that the world is still out there – real, she says. Jane moves around a bit, touches objects, looks at the windows, smells the soap perfuming her hands. I feel myself on the outside of the glass containing her experience. But is there a way for her to reach back out?

Recovery from an episode can be as imperceptible as the onset is sudden. The weeks and months after France were a carousel of doctors, and different diagnoses from hayfever to neurological conditions and post-traumatic stress disorder. But not having your primary symptom properly acknowledged is very distressing.

About two years in, my therapist suggested I see the in-house psychiatrist. He was the first person who described my symptoms as ‘depersonalisation’ and ‘derealisation’. I’d never heard these words before – but the instant he said them, I knew I had a diagnosis. It didn’t make me better, but it gave me a vocabulary and it de-mystified the condition to some degree.

There were two other vital components to that first period of recovery. The first was to distract myself – if I was constantly engaging in something interesting or funny or challenging, then I could find a bit of space away from the depersonalisation. The other was to watch my own thought processes, and try to let go of the most destructive ones. This is harder than it sounds, since those thoughts dominate your every moment. It takes perseverance and discipline.

Jane returns to the table, temporarily reassured that the world is still there. Her account of living with DPD suggests that even if the self turns out to be an illusion as a matter of science, its presence to us is what hooks us into the world. If the self is such a vital component of what it even means to experience reality, philosophers should be cautious not to slight its significance.

I still have the memory of what it’s like when the depersonalisation lifts. Those are periods of such indescribable joy. They’re memories I try to hang on to when things get tough – memories of just sitting at my tiny kitchen table in my flat, without feeling the need to achieve or function or engage. Just being. Just living.