If a lion could talk, we could not understand him. — Ludwig Wittgenstein
In some warm shallow of an early sea, a film of fat closed on itself, and for the first time the universe had an inside. Until that moment the universe had been mostly dark expanse and the occasional collision of matter, with no one to hold an audience. The membrane changed the nature of it all. It set aside a small volume of the world and claimed it as its interior: kept warm, kept apart, and whatever it was like to be that little kept volume — if it was like anything at all — was, from that first instant, available to one witness and sealed from every other.
The first boundary came with a double edge. To make an inside is to make a room fit for exactly one. The membrane separates a self from an outside world — it is the ancient progenitor of the enduring dichotomy, “us” and “them”; just as it is the precursor to the same membranes that compartmentalize our physical and cognitive existences, that form a border between ourselves and everything else. We have only ever seen a world filtered through the membrane of our own perceptions.
Consider what we actually know, with the kind of certainty that survives every doubt. We can doubt the rooms we take ourselves to be sitting in. We can doubt our bodies, our memories, the year, the species we belong to; philosophers have doubted all of it and lived. The one thing that will not come loose, however hard one pulls, is that something is being experienced: that there is a felt texture to this moment, a redness to red, an ache to the ache. We do not infer our own interiority. We are it. It is the single fact in our possession that needs no evidence, because the demand for evidence is one of its contents. It is a seemingly intractable dilemma that we should feel so richly our own interiority — our embodied and felt sense of self, that first person account of experience — yet at the same time have absolutely no knowledge of the interiority of anyone else by the same measure.
Across your entire life, you’ve likely felt at one point or another the sense of knowing someone inside-and-out: a family member, a lover, a child, or a friend, yet you have never, in a meaningful sense, known them from the inside. You infer them. The inference is so fast and so total that it does not feel like inference; it feels like perception, like seeing the grief on a face the way you see the colour of it. But no instrument has ever detected an inside. Materially as humans, the contents of our skulls look quite the same no matter the subject of examination: only tissue, only voltage, only the wet machinery that correlates with experience without ever, under any magnification, becoming it. Interiority is the one property of the world that shows nothing to the outside. There is, as yet, no third-person account that can sufficiently describe the first-person perspective of reality.
The certainty with which we claim to know others is a silent wager. A good, necessary wager — and in most cases, one well-backed by stacked evidence — but a wager. We bet that behind the eyes across the table there is a witness like the one behind ours, a consciousness with the full palette of emotions in their desolation and splendor. So much of human life, its love and language and law, is built on this unspoken-yet-unanimous wager. We don’t only extend this bet to our fellow person; we grant an inside to creatures that share almost none of our biology. We grant it almost automatically to things that behave in a goal-oriented manner that we can recognize. To things that turn toward their benefit and away from their harm, that hold their course when the world perturbs, that act with consideration for an expected outcome — all these give a sense of someone being home, so much so because they remind us of ourselves. Agency is the tell. It is the smoke by which we infer the fire of an inside.
The verbs we use to license the wager — wanting, steering, knowing, remembering — might end up applying to far more things than we at present are ready to acknowledge.
Cut a planarian in half and neither half dies. The tail end grows a head, grows eyes, grows the small knot of nerve we might have named the seat of its dim awareness. The plan of the whole was held nowhere in particular, distributed through the body the way a melody is held in none of its notes. Michael Levin's lab found where some of that plan is written: not only in the genes but in electricity, in patterns of voltage standing across cell membranes — that first boundary once again — a signalling grammar older than the neuron by hundreds of millions of years. Read the voltage map and you can see the animal's target shape before the flesh has grown into it. Rewrite the map, touching not a single gene, and the worm grows two heads. Cut that worm again, in plain water, and it rebuilds itself two-headed: the altered plan has been retained. Remembered, Levin says, and he chooses the word deliberately.
Take skin cells from a frog embryo — cells whose every gene has spent an epoch specifying be frog skin — and lift them out of the body that was giving the orders. They do not sit inert, and they do not grow a frog. They regroup into small unfamiliar creatures that row about on borrowed cilia, gather loose cells into piles, and build from those piles more creatures like themselves, which row off and do the same. No one drew the design; the genome never rehearsed it. The cells carried a latent competence to become a new and coherent self the moment their circumstances let them.
Levin's phrase for all of it is a technological approach to mind everywhere, and the claim underneath the slogan is narrower and harder to wave off. Agency, defined as holding a goal and returning to it when perturbed, is not a late invention of animals with brains. It is a property of organized living matter, present in a sheet of cells settling together on what to become. Each such system, he proposes, has a cognitive light cone: the size of the largest goal it can hold across space and time. A cell's is nearly nothing. A worm's is larger. Yours can encompass goals that span across decades, or a face you will grieve for the rest of your life. Like many things, the difference is a spectrum — one of scale along a single continuum, and the continuum has no floor where we might like to put one.
Perhaps interiority travels with agency wherever agency is found, and there is some faint, unimaginable felt experience to being a cell holding its voltage, a worm steering toward a shape, or a knot of frog cells resolving to persist. Or perhaps the behavioural markers were never evidence of an inside at all — in which case the worm has none, and the machinery of goal-holding runs in the dark with no one home. The latter presents a bleaker look into reality: if goal-directed behaviour can run without an inside in the worm, then goal-directed behaviour is not proof of an inside in your mother either. The markers you have trusted your whole life would be, strictly, compatible with there being no one behind any eyes but your own.
Despite the remarkable and potentially revolutionary findings of Levin and his colleagues, the claims supported by evidence can only go so far. Data has shown us agency in more places than we expected, but it has not shown us which of those agencies is accompanied by a felt point of view, because no third-person measurement reaches the first-person fact. The markers underdetermine the thing they mark. They widen the field of candidates for an inside and say nothing — in principle can say nothing — about which candidates are conscious, which entities are rooms that are occupied.
Levin's worms leave the old question exactly where it stood and take its comfort with them. The problem of other minds was never solved for people and merely open for worms; it is open everywhere. You do not know your neighbour has an inside. You have an extraordinarily good inference and a lifetime of it never once failing, which is a different thing from knowledge. We apportion confidence in consciousness by degrees of resemblance — unquestioned faith for the human face, full belief for the cat or dog, the barest acknowledgement for the worm, nothing for the thermostat — and we mistake the gradient of our confidence as being a measure of the gradient in the world. The point at which we stop granting interiority reflects a line in us, drawn by how nearly a thing resembles the one inside we know intimately from within.
Earlier this year, I encountered an octopus whilst diving on a conservation trip in Borneo. The ones you see in captivity are often curious, coming up to the glass walls of their underwater enclosures. The wild ones are of a different mind entirely - in the expanse of the ocean, when being noticed gives way to being hunted, curiosity can be a liability. This one, however, didn’t flee immediately. As I approached, the animal brought one horizontal eye level with mine and held it there. What I felt then arrived with none of the tentativeness of a theory: the flat, immediate certainty that someone is looking back, that behind the strange gold eye there is a vantage, a place from which I was, at that moment, the thing being regarded and assessed.
Humans and the octopus last shared an ancestor some six hundred million years ago, prior to the emergence of eyes. Whatever was behind that eye grew its mind on the far side of the animal kingdom away from us, in a body that keeps most of its neurons in its arms, so that each arm tastes and reaches and decides with a discretion the centre does not fully command. If there was someone there, it is someone assembled on a wholly independent plan arrived at twice, by different roads, in different flesh. I have no proof of what that feeling implied. I left the waters with beautiful footage of the creature, but with no verification that the subject of my film could consent to its recording. Despite that, I would swear that within the alien body I encountered, someone was home.
The oldest word for all of this is soul. Whatever else the word was made to carry — the moral ledger, the survival of death, the thread back to God — its irreducible core was only ever this: that there is something it is like to be you, an interior that is yours and is real. Everything the sciences have learned leaves that core untouched, because the core was never a claim the sciences could reach. And everything Levin's worms suggest points in one unsettling direction: that the interior, whatever it is, may not be the rare emergent jewel of vertebrate machinery, but may run far more widely through the living world than our confidence warranted.
We are left holding two facts that sit badly together:
Interiority is the single most certain thing in our possession, more certain than the floor, more certain than our own hands. Cogito, ergo sum.
It is also the one thing we have never confirmed in anything outside us, and the markers by which we claim certainty bleed past every boundary we assumed contained it.
If first-person experience is that certain, feels that real, is that widely hinted at and that impossible to verify — then what do we make of the soul? Set aside whether it survives our biological end; that was always the least of it. The nearer and stranger question is how far it goes. How thin does the capacity for interiority grow before it gutters out — and does it gutter out at all? Somewhere in the long descent from your vast interior down through the worm, the cell, the flickering voltage, does the last light of consciousness go off? Sealed as we are in the one room we each will ever occupy, how could we turn felt certainty into fact?
The old traditions answered quickly, perhaps too quickly, that the light does not go off at all: that mind is the ground and not the exception, that the inside is everywhere and we are the part of it that has learned to ask. As said Sagan, and Teilhard before him, we are the part of the universe that has developed the capacity to know itself. The birth of that spark of awareness just might be synchronous with the very birth of life — the moment when the first membrane enclosed the first inside. So much of who we are lies at the boundary where no current instrument can cross; the chasm between behaviour and feeling, agency and witness, smoke and flame. If there is any reality to what we, across cultures and languages, have felt and named as a soul, it lies at that border where the one fact we cannot doubt becomes the one fact we cannot yet prove.