Buddhist interpretations of personhood suggest that we have a misguided understanding of our own internal reality; we perceive ourselves as part of a continuous state of being, moving from day to day and year to year. You, my friend, are a constant. An individual. Self-awareness is self-evident, right?

That perception, however, may be wrong. Personalities can change as we walk through life. One day you’re happy, the next, angry at losing your job. That anger can change you. Fear can change you, and some Buddhists would argue that you’re really a new person from moment to moment, with memory providing only the illusion of continuity. Events can set us free, scar us temporarily or permanently. Memory is not just a personal narrative, it shapes who we are. We recall our memories and reflect upon them, and that mere act of reflection and evaluation can alter our choices and, thus, who we become.

So, in light of that, consider this man: Henry Gustav Molaison, for years known only as Patient HM. Back in the 50s, HM suffered from intractable seizures, painful episodes that made life unbearable, and it was concluded, after careful evaluation, that surgery was his last, best option. So, on September 1^st, 1953, Doctor William Scoville opened up HM’s skull and cut out large swathes of his temporal cortex, his amygdala, his hippocampus—all brain areas that deal with types of memory.

His epilepsy was gone, sure. But with it, also, went a crucial part of his, or anyone’s, personality: the ability to create and update the narrative of his life.

Anterograde Amnesia is the name of the condition.

Think Memento. HM could remember things almost up to the “event”—his surgery—but he was completely bereft of the ability to form any new memories afterwards. He was stuck, mentally, in a moment with no way of moving forward. He could remember what his wife looked like when they married but couldn’t recognize her decades later. He was a stranger in the mirror. He could hold information in his short-term memory for a few moments, perhaps minutes if not distracted, but any longer and it would just evaporate. And if he met you, he couldn’t recall it, not even if you re-introduce yourself to him ten seconds later. Or ten seconds after that. Or after that.

Declarative memories, or memories you can tell somebody about (e.g. episodic: “I was at the library.” Or semantic: “Delaware was the first state.”) were a problem as well, but previously learned motor skills like riding a bike were fine, clearly demonstrating there is no monolithic entity known as “memory” that is controlled by a single brain area, or even a handful of them.

Back then it was thought that HM’s deficit was due solely to hippocampal damage, but obviously we’ve learned that isn’t the whole story. The hippocampus, a sea-horse shaped structure in the brain, is needed to form new memories and access some older memories, but many of those “long-term” memories are actually stored in numerous cortical areas as widely dispersed “neural nets.” There isn’t a specific physical location, per se, that corresponds to, say, remembering your first kiss. Stimulation of a specific brain region can trigger the memory, but likely because of activation of a distributed network of brain cells that all talk to each other.

As for destroying past memories, now that’s a whole other story. Retrograde Amnesia, which makes the science fiction and noir genres so great, isn’t too hard. So what if that rat has run the maze for months? Bam! Lesion—the rat has no clue how to find the cheese. We can cause some rough memory deficits similar to HM’s in animals, too. We can even do it reversibly by temporarily cooling a brain region, or by using Lidocaine or other drugs that wear off instead of removing brain parts which, let’s face it, can be a little messy.

For long-term memory suppression, there’s the drug Propranolol, which is used to treat anxiety and Post Traumatic Stress Disorder and is sometimes called the “amnesia drug.” That name obscures the truth a bit, though, since Propranolol doesn’t actually make somebody forget fearful things, it merely decouples emotions from events. You remember the trauma, just not the fear or anger that accompanied it.

Loss of one’s past, that Retrograde Amnesia that ironically never gets old as a plot device no matter how many times it’s used, is trickier, especially as we move towards targeted memory erasure. To be certain, people who experience traumatic events can have Retrograde Amnesia for big chunks of memories, but what if you just want to excise specific parts of your past? What if you want to pull an Eternal Sunshine of the Spotless Mind and get rid of memories of that ex-girlfriend who broke your heart, for example?

First, you need a way of mapping which brain areas are active and correspond to certain undesirable memories. Right now we can do that down to a couple cubic millimeters of tissue. But that’s not enough. We would also need to map out activity of individual neurons, or, if possible, the activity of individual connections between neurons called synapses. But to give you an idea of how tremendously horrifying such an undertaking would be, there are between 10 and 100 billion neurons in the brain. Each neuron can make thousands of connections, and each connection can fire hundreds or even thousands of times a second. So…good luck with that.

Now, once this “simple” task of creating activity maps is done, we would then need a way to inhibit patterns of activity that correspond to undesirable memories. The problem here is that inhibiting those activity patterns with, pardon the pun, surgical precision might be next to impossible, at least at the synaptic level. We would have better luck wiping out cells or groups of cells, and as they say in Eternal Sunshine, technically that procedure is brain damage.

However, that still doesn’t lend itself to a spot-free mind. The more damage you do, the more your other memories will be affected. You didn’t just have a girlfriend, you also had friends in common. You went to favorite restaurants. You hung out with each others’ families for the holidays. Those memories won’t be unaffected. You won’t have a nice, clean ex-girlfriend shaped hole in each of those memories. You remove a painful memory, you leave the other memories around it all torn up and uncomfortable, because there’s no context in which to find understanding. The ones that are left have gaps that don’t make sense. So maybe those should be removed as well. Add, rinse, repeat.

And so now, here you are. You’ve got one memory left. It’s peaceful, this one. You’re a child, sitting on a beach, watching the sun go down. But this single, isolated memory, nice as it is, tells you nothing about who you are. The person you were is gone, lost at the end of a breadcrumb trail of burnt-away pasts. Good, bad, smart, dumb, all you’ve got now is your physical self and that sunset. Tabula rasa, baby.

And while, yes, the loss of all one’s memories is a nightmarish scenario to contemplate, if, like the Buddhists say, you really are a new person from moment to moment, is it such a total tragedy? You still have the ability to create new memories, right? Become a new you, write a new narrative? The action of life is forward moving. Dwelling in the past: that’s for suckers. And in that truth, perhaps, is the essence of what makes us human. Kinda profound, huh?

Now consider, once again, Patient HM. His past memories remained largely intact, and in that way, at least, he remained himself. But what he did lose was the ability to evolve from that place. He simply no longer had the machinery to change, to become a new man. And no longer having that ability to learn, to grow, to create your own future? Well, that, too, is profound.

Profoundly disturbing.