There is a tradition in philosophy that makes our memories the core of our personal being, essential to what we are, and the basis of our personal identity over time. This stems from John Locke’s discussion in An Essay Concerning Human Understanding (first published in 1689), and his famous thought experiment where a prince and a poor cobbler wake up with memories exchanged. According to Locke, if the cobbler (who now has the prince’s memories) had committed a crime, we should hold the person with the prince’s body (and the cobbler’s memories) responsible, and vice versa. For Locke, memories and memory continuity make us who we are, and it is only a contingent fact that our memories are usually tied to a particular body over time. He even contemplated the possibility of there being more than one person within a single body – a person who lives by day and one who lives by night, for example, with their memories entirely encapsulated, the one opaque from the other, unaware of each other’s existence. On a Lockean view, if we lose enough of our remembered continuity, perhaps through dementia, then we literally become a different person, or perhaps not even a person at all. And if we really can’t remember our moral failings or crimes, we shouldn’t be held responsible for them because in an important sense we didn’t do those things – same body perhaps, but a different person.

Probably many of our memories aren’t so easily separated from aspects of a particular body. Memories of how to do things, rather than of facts or past events may be far more embodied than Locke and his followers suggest. I’m thinking of memories like the memory of how to drive, or ride a bicycle, or play a musical instrument. And character traits of warmth or generosity can persist even when individuals have almost completely lost a sense of who they are and can’t recall key events of their past. There is a sense in which something of a person can remain even when their mind has been reduced to almost nothing by memory loss. As we grow older, we don’t only increase the risks of gradually losing ourselves and those close to us through the memory loss and perseverations of dementia, but we also risk losing our own memories when those dear to us die. They can take our memories with them when they die; not just their own. Philosophy can shed some light on this.

It is twenty-five years since Andy Clark and David Chalmers published their first paper on the extended mind thesis. This rich hypothesis suggests a way of understanding what we are that can clarify what it is we stand to lose when our friends and family die before we do. We can lose not just a person dear to us, but potentially our own memories, which are part of what we are, even though it was in someone else’s brain that the memories were held. Clark and Chalmers originally wrote about how someone whose memory was unreliable but who used detailed notes to himself as a way of navigating a city could be thought of as using his notes as an external memory. When they used the word ‘memory’ here they did not mean this as a metaphor. In place of the prejudice that the mind resides within the skull, or at least within the individual’s body, they presented a picture that allowed us to make parts of the world parts of our minds. The argument rests on the function of the external tool or memory device, its readiness to hand, and its reliability.

The philosopher of mind Ned Block quipped that this view of the mind was false when first stated but has subsequently become true – because of the advent of the smartphone. Smartphones hold our memories of telephone numbers and addresses, and much else besides, and they can plausibly be thought of as extensions of our mind, even though saying the same about a notepad may have been seen to be more like a metaphor back in 1998.

Some philosophy leaves everything as it was, but simply redescribes it so that we see it in a different way. This is what Ludwig Wittgenstein was getting at when he wrote about ‘the dawning of an aspect’. He used the puzzle picture of a duck-rabbit that could be seen either as a duck or a rabbit. To see it as one not the other required no new sensory input. It was the result of a Gestalt shift – the same information perceived differently. This is very much how I see the extended mind thesis. We haven’t learned something new from it, rather it is a catalyst to think about what we already know in a new way. Chalmers has described this as being like a Necker Cube. Previously we might have seen the mind as basically within the head; now we can see the mind as potentially including the items that we carry with us that extend our memories and other sorts of perceptions of the world. This has great explanatory force.

My memory for past social events is not capacious or reliable. There are parties and dinners and meetings that I can’t easily recall or distinguish between when I try to recall them. My wife, however, remembers the ones we’ve attended together with a startling accuracy. She can recount who was there, what they said, what they were wearing, where we were. Sometimes it’s uncanny how much she remembers and a little disturbing how much I have forgotten. Within a couple it’s quite common for there to be some division of labour – in this case, she is far better than me at preserving our joint memories of occasions like these. Once she gives me a few hints, something of the memory returns. Without her memories, mine would be irretrievable and a blurry mélange of multiple occasions. One way of thinking about this is to say that I have outsourced some of my memories in her memory.

The extended mind thesis explains why the death of someone very close to us, with whom we have many shared memories, can be felt as such a loss. We don’t just lose what we value about that person and the possibility of future meetings and interactions, we don’t just lose the person we valued and loved; we also lose something of ourselves. When parents or spouses or very dear friends die, they take a part of us to the grave with them. We really do die a little when they die, and we are quite literally diminished in the process.

From the Loss edition of New Philosopher, which you can purchase from our online store. Subscriptions to the world's leading philosophy magazine can be purchased here.