Can you lose your identity while still being alive? Yes. I saw my mother lose hers, then regain something of it. One day she could do The Times crossword puzzle. A few weeks later she began to worry that there were small people crawling over the food in the supermarket. Only weeks after that she had forgotten how to speak, how even to stand up and sit down. She was a shell of herself. She couldn’t communicate and was detached from what she had been. Drugs and time brought back some of her personality and memories, albeit intermittently and in a confused way. But vascular dementia is cruel. She never really came back, was never again the person that she had been, apart from, perhaps, when she smiled. She confabulated to create a consistent story of what was happening and to make sense of her surroundings, but she was adrift from reality, and often incoherent.

In the 17th century, John Locke famously argued that what makes each of us the same person over time was a particular kind of continuity of memory. He used ‘man’ and ‘person’ as technical terms, suggesting that someone could be the same man over time, but not the same person. In Locke’s sense my mother was the same ‘man’ (meaning member of the species homo sapiens) as she was before her dementia, since there was biological continuity – her body was the same body, despite significant changes to her brain and other parts. She was the same ‘man’ as she had been seventy years earlier too, again because of this continuity. We change, grow, decline, just as an oak tree does while remaining the same oak. But Locke was most interested in the question of personal identity: what made someone the same person over time was something that could only arise for a conscious being.

For Locke, a person was “a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing in different times and places”. In order to have personal identity, I need some kind of self-awareness, an ability to think of myself as continuous with my past, having more than just bodily identity. The key here is a recognition of my own past actions as something for which I was responsible, as mine – a set of interwoven memories of being me.

Were a prince to wake up with all the memories of a poor cobbler, and a cobbler to wake up with all the memories of the prince, Locke argued that we would, and rightly should, consider the individual with the prince’s memories (and the cobbler’s body) responsible for whatever the prince had done, and the one with the cobbler’s memories (and the prince’s body) responsible for whatever the cobbler had done. That’s how we would see it if we had God’s view of what was going on.

Obviously, this thought experiment doesn’t describe anything ever likely to happen. The point of it was to emphasise that personal identity is not simply bodily identity, and could in principle even be separated from it. What makes each of us a person is our memories of our selves and of our place in the world, what we have done, what we have thought about doing, who our friends and family are, what we care about. The filmmaker Luis Buñuel captured this eloquently in his autobiography:

“You have to begin to lose your memory, if only in bits and pieces, to realise that memory is what makes our lives. Life without memory is no life at all... Our memory is our coherence, our reason, our feeling, even our action. Without it we are nothing.”

For Locke, to be the same person as the child that was evacuated from London to the Wiltshire countryside during World War II, my mother would have had to remember that momentous event as part of her life, as something she experienced. But the philosopher Thomas Reid writing in the 18th century raised a difficulty for Locke’s account. Imagine a soldier who was once flogged at school for stealing apples from an orchard. In his first campaign as a soldier, he captured an enemy standard. At that point he could remember being beaten as a child, and later when made a general he could remember capturing the standard, but could no longer remember the incident with the apples and its aftermath. For Locke this would have to mean that the boy was the same person as the young man, and the young man was the same person as the old general. But, also from Locke’s account, because the old general’s memories didn’t extend back to his childhood, he wasn’t the same person as the child. That gives the contradictory conclusion that for Locke the general both was and was not the same person as the child (although he was clearly the same man in Locke’s sense of the term). So, something was clearly wrong with Locke’s account. Perhaps Locke set the bar too high, and all we need as a criterion for personal identity is some sort of sequential overlapping pattern of memories of the kind Reid described.

Both Locke and Reid wrote as if memories were necessarily accurate. Yet we now know much more about how memory functions, and the most plausible accounts stress how much memories are changed by the act of recalling them, and also the fundamental unreliability of testimony. This complicates things. My mother’s memories of being evacuated in wartime would have been transformed by telling her children and grandchildren about them, and the new versions would then have become her memories.

In a sense, we are all severed from the reality of our past, to varying degrees, because of the way memory typically works. And our memories are unstable. It’s not as if we go to a filing cabinet and access a folder of accurate first-person accounts together with photographs taken from our point of view. Rather, each time we get our documents from that filing cabinet we end up putting something a bit different back into the folder, and sometimes we even build in other people’s accounts into what we take to be our own memories.

It must be right, as Buñuel knew, that, despite this, our memories and our consciousness of being a person make us who we are, give us our continuing identities and apparent coherence. It must be right, and we know it is right because we see the loss that dementia can inflict. We rightly, at a certain point, think of substantial memory loss as a loss of personal identity. But if our memories are so unreliable and in flux, we are more like creative storytellers of our identities than we usually realise or want to admit. We are all confabulating to some degree, some much more than others, and the memory-based stories we tell ourselves and other people about who we are and have been, and how we have become what we have become, are neither particularly reliable nor stable. But that’s all we have, and if that all goes, we really have lost our selves.