When my father died, our dog took to sitting by the front door, waiting for his master to return from his absurdly long walk. To me too, the fact of his death felt impossible. I knew it was true, but it took me so very long to believe it. Most bereaved people experience similar feelings, and it turns out there’s a reason for that.

Mary-Frances O’Connor, author of The Grieving Brain: The Surprising Science of How We Learn from Love and Loss, says: “When people say, ‘I know they’ve died, but it feels like they’ll walk through the door again,’ I think that’s an accurate description of how the brain works. We can hold two beliefs at the same time.

“On the one hand we know rationally they have died. We have memories of caring for them, or being at the funeral,” says O’Connor, who is an associate professor of psychology at the University of Arizona, where she directs the Grief, Loss and Social Stress Lab, which investigates grief’s effects on the brain and body. “But on the other hand, what we know about attachment is, when we form a bond with that one and only, part of what comes along with that is the belief that they will always be there for you – and you will always be there for them. And that belief persists.”

That belief is what makes it possible for us to kiss our partner goodbye in the morning and go our separate ways – our brain, that predictive machine, knows implicitly that we’ll reunite. So, she says, when “a person to whom we are so tightly bonded dies, it doesn’t make sense to the brain, because of this deep-seated belief”. (We can subconsciously feel as if we’ve been “ghosted” – and feel angry at being abandoned.) “And yet, we have this other stream of information that tells us it’s true. So the disorientation of grief is because you’re trying to reconcile these two forms of knowledge – and why it takes so long.”

We often say after a loved one dies that they stay in our heart – but science says it’s more than that. Our belief in their existence is so innate that in our mind our relationship is never-ending, notes O’Connor. As a consequence of loving them “the neural connections... for the mental representation of our loved one are permanently encoded”.

She herself has borne the loss of both parents. In the early months after her mother’s death, she stuck a note on the kitchen cupboard that read: “Cook. Clean. Work. Play” – to refer to when she felt overwhelmed, and to remind her that even the smallest accomplishment was enough.

It gives her some consolation, she says now, “as a neuroscientist, knowing that the proteins in my brain are folded differently because I had this relationship. Because they loved me – and that I continue to carry that, even after their death physically, I carry that – that ‘we’ part of us – is very comforting to me.” And yet the pain of grief can feel unmanageable. When we ask, “How can I live without this person?” we mean it. To the scientist, this makes perfect sense. “I think we forget,” she says, “that the closest people to us, are as important to our survival as food and water. If you think about how you would feel if there was no food, or only a tiny amount of food, it helps us to see why it’s so intense when we lose a loved one.”

When we meet “the One,” there truly is chemistry. “The neurochemistry in our brain and our body stimulates, and is stimulated by, falling in love,” notes O’Connor. And when our beloved dies, “all our neurochemistry – all the dopamine, the opioids, the cortisol – are motivating us to seek out this person again”. A vital element is lacking, and to the brain the solution is obvious: “Go get them.” And it is very painful, she adds, when the solution, so important to your survival, “simply does not happen”.

It’s not unusual for a bereaved person to feel as if a part of them is missing – and this isn’t poetic licence. O’Connor says that while, of course, in many ways we function in the world as individuals, “our brain also thinks of us as part of a larger whole – as a couple, or a parent and child – and I believe, it encompasses the two of us as a functioning unit.” And so, “when a piece of that is taken away, I think the experience of it isn’t just that, metaphorically, a part of us is gone, but there are enough suggestions in neuroscience that it feels that physically a part of us has been taken away as well.”