The cumulative, probabilistic case for modal realism: why do we exist?
How probability, parsimony, and a brutal honesty about reality point to modal realism
Imagine waking up in a vast and unfamiliar desert. You do not know where you are, or why you are there, or what anything means. The only certainty is uncertainty itself, but you notice some things: the sun always rises from the same direction, the wind leaves repeating patterns in the sand, some plants heal, others kill. By testing, failing, and adapting, you begin to map the landscape, not perfectly but better still. You throw away the maps that lead you into pits and keep the ones that bring you water. This, in a nutshell, is science. It is also the method that guides my philosophical thinking, and it is why I argue, probabilistically and cumulatively, for modal realism.
Let us begin with a basic observation: what we call "seeing" or "experiencing" is never about accessing something intrinsically "visual" or "material" out there. We see with photons, but there is nothing essentially visual about photons. It could have been so different. We could have evolved to "see" with magnetic fields, or with neutrinos, or even with photons but in ways completely alien to how we currently see. Visuality isn’t the experience of something visual out there, it is the very experience. Photons are no more visual than neutrinos, electrons, or quarks.
By generality of this argument, it can be readily extended to all of our senses, to every shred of so-called "physicality" itself. There is nothing inherently tactile or solid about atoms (it is noteworthy here that if an atom was enlarged to fit a schoolyard, the nucleus, which is the tiny core of the atom, would be about the size of a small marble, and most of the atom would be empty in the "containing substance" sense), nothing fundamentally auditory about vibrations in air; all such correspondences are contingent mappings, arbitrary alignments in a sea of possibility-relation-space.
This is, perhaps, an incomplete, but robust bridge from the concrete to the abstract: if physicality itself is reducible to abstract relational structure, then the boundary between what is "real" and what is merely "possible" begins to erode.
It is here, standing at the vanishing edge of substance and possibility, that we encounter the familiar yearning for an anchor, a final cause, a cosmic author, something to declare: "this is real, and that is not." For most of history, this yearning took the shape of God. The invocation of a supreme being was meant to halt the infinite regress of explanation, to deliver a necessary foundation under all the shifting sand. God was the answer to why anything exists at all, the ultimate selector from the garden of possibility.
But here too, under the same honest scrutiny that strips "physicality" of its privileged status, the idea of God dissolves into arbitrariness. The notion of a creator, a cosmic decider, does not answer the riddle of existence, it merely hides it one step deeper. For if God selects this world, why this God, and not another? Why does the "world" of God exist, with its particular set of properties, psychology, and universe-making preferences, rather than some other order, or none at all?
The devout religious will protest, of course. Any isolated challenge to God’s existence can be dodged by way of theological gymnastics: the problem of evil is explained away with free will; the absence of miracles is attributed to divine hiddenness; the incoherence of omnipotence is buried beneath paradox and wordplay. In isolation, each objection is met with a ready-made excuse, as if apologetics were an endless game of rhetorical whack-a-mole. Together, they form a probabilistic mountain against God where the parts self-reinforce each other in ways far beyond their sum. Simply put, the improbable, when multiplied by the improbable, becomes even more improbable.
Likewise, the reducibility of physicality to abstract structure, the arbitrariness of any supposed "selector," the infinite regress of world-choosing mechanisms, may seem like mere speculation. But together, they form a probabilistic mountain where they reinforce one another in a way that makes any rival explanation look baroque, special-pleading, or simply desperate.
A common challenge to modal realism is Occam’s razor, which dares to declare "entities should not be multiplied beyond necessity" but contrary to popular misunderstandings, it does not simply favor simple-minded explanations that wave away worlds or possibilities just to avoid messiness. It is a scalpel, not a chainsaw. Occam’s razor trims away unnecessary complications, not ontological multiplicity demanded by the logic of our predicament. To posit a final selector or a privileged world without reason is the very bloat that parsimony is meant to excise. Modal realism, for all its grandeur, is not extravagant for extravagance’s sake, it is simply what remains when every ad hoc fix, every cosmic loophole, every special pleading has been stripped away.
If the data, the totality of philosophical and empirical pressure, requires modal realism, then so be it. Multiplicity is not a crime if it is the only honest account left standing. In contrast, if you want to posit some "selecting" principle or entity that chooses which possible worlds are real, you quickly find yourself in a hall of mirrors. Either you propose an infinite regress of selectors, each picking the last, or you demand a final selector-world, with no explanation for why it, in particular, got the job when surely there were many other possible final-selector worlds. In a nutshell, far from eviscerating modal realism, Occam’s razor eviscerates all the alternatives.
Another common challenge to modal realism is that it is simply too unintuitive, too alien to how we instinctively picture reality. This objection is said as if intuition is some kind of divine barometer for metaphysical truth, a mystical sixth sense that could never steer us wrong. But intuition isn’t magic. It is simply the end product of evolution’s blind algorithm. It is shaped, and hopelessly limited, by the narrow band of data our senses deliver, a trick of survival, not a path to ultimate knowledge.
If our intuitions feel violated by the vastness or indifference of modal realism, that is not an indictment of the theory, but a reminder of our own parochial perspective. The universe owes us neither comfort nor agreement with our gut feelings. Intuition is only as reliable as the quality and scope of the information it digests, and the universe, as it turns out, is not obligated to resemble the inside of a Paleolithic brain.
The fact that you happen to be in this part of the desert, and not another, is a trivial fact, important only to you, and perhaps not even that, depending on how long you last before the sun finishes its work. Your patch of sand feels real because it is what you know, but that sense of solidity is merely local, an accident of vantage, not a cosmic distinction.
Consider, then, in the endless space of possibility, another desert, one made not of your atoms or fields, but of what you might call "ghost" particles. To you, these particles are quite literally nothing; they do not interact, they cannot be seen, measured, or even represented as part of your physics. They are, for all practical, non-existent. With respect to you, your matter, and your laws, they are pure absence, as if a blank space in the catalogue of all that is. Their "ghostliness" is not some poetic vagueness, but the strictest definition of absence: they have no reality except for themselves.
Yet, from the perspective of the ghost particles themselves, it is quite the reverse. Each ghost particle exists only through its relations to other ghost particles; there is no external world, no absolute backdrop, nothing beyond this network of internal definition. For them, "to be" is nothing but "to be related." Each particle’s properties, history, and even its individuality are simply patterns of connection within the ghostly system. Their existence is entirely self-contained, self-justifying, and closed. The ghost world is not an imitation or a lesser reality, it is, for itself, a complete and internally coherent domain.
If the web of ghostly relations is deep and complex enough, if within this self-referential network, patterns of sufficient complexity and stability arise, then something remarkable happens. Structures emerge that can encode memory, process information, adapt to novelty, and reflect upon their own patterns of relation. Out of the ceaseless dance of relationally-defined ghost particles, a ghostly consciousness will awaken, experiencing the shifting landscapes of its world with the same immediacy, depth, and longing that you feel under your sun.
In that moment, the boundaries between substance and absence, between reality and mere potential, dissolve entirely. For this ghostly mind, every sensation is as acute, every desire as real, every sorrow as deep as any that ever tormented flesh and blood. The world it inhabits unfolds for it with total presence. Each feeling and thought arises within a fabric of internal relations that define what it can know, remember, and hope for. At last, the ghost mind moves through its desert, testing the ground beneath it, seeking meaning in patterns and cause in events, not knowing its world is invisible to all but itself.
Reality, finally seen clearly, is always defined from within. Each network of relations, wherever complexity and coherence allow, creates its own world, a center of experience no less real for being unreachable from beyond its closed horizon. Every world whose parts are defined with respect to each other is, by that very fact, real for itself. To everything outside, it is nothing; but to itself, it is everything.