Suppose our old friends Alice and Bob decide to undertake an art project. Alice will draw a bunch of random purple and green lines on a piece of paper. That will be Alice’s picture (A). She’ll then make a copy, erase all the purple lines, and send the result as a message (M) to Bob. Bob then generates his own random purple lines, and adds them to the green lines from Alice, to create Bob’s picture (B). The two then frame their two pictures and hang them side-by-side to symbolize something something similarities and differences between humans something. Y’know, artsy bullshit.

Now, suppose Carol knows the plan and is watching all this unfold. She wants to make predictions about Bob’s picture, and doesn’t want to remember irrelevant details about Alice’s picture. Then it seems intuitively “natural” for Carol to just remember where all the green lines are (i.e. the message M), since that’s “all and only” the information relevant to Bob’s picture.

In this example, the green lines constitute a “natural latent” between the two pictures: they summarize all and only the information about one relevant to the other.

A more physics-flavored example: in an isolated ideal-ish gas, average energy summarizes “all and only” the information about the low-level state (i.e. positions and momenta of the constituent particles) at one time which is relevant to the low-level state at a sufficiently later time. All the other information is quickly wiped out by chaos. Average energy, in this case, is a natural latent between the gas states at different times.

A more old-school-AI/philosophy example: insofar as I view dogs as a “kind of thing” in the world, I want to track the general properties of dogs separately from the details of any specific dog. Ideally, I’d like a mental pointer to “all and only” the information relevant to many dogs (though I don’t necessarily track all that information explicitly), separate from instance-specific details. Then that summary of general properties of dogs would be a natural latent between the individual dogs.

Just from those examples, you probably have a rough preliminary sense of what natural latents are. In the rest of this post, we’ll:

• Walk through how to intuitively check whether a particular “thing” is a natural latent over some particular parts of the world (under your intuitive models).
• Talk about some reasons why natural latents would be useful to pay attention to at all.
• Walk through many more examples, and unpack various common subtleties.

Unlike Natural Latents: The Math, this post is not mainly aimed at researchers who might build on the technical work (though they might also find it useful), but rather at people who want to use natural latents conceptually to clarify their own thinking and communication.

We will not carefully walk through the technical details of the examples. Nearly every example in this post has some potential subtleties to it which we’ll gloss over. If you want a semitechnical exercise: pick any example in the post, identify some subtleties which could make the claimed natural latent no longer a natural latent, then identify and interpret a natural latent which accounts for those subtleties.

What Are Natural Latents? How Do We Quickly Check Whether Something Is A Natural Latent?

Alice & Bob’s Art Project

Let’s return to our opening example: Alice draws a picture of some random purple and green lines, sends only the green lines to Bob, Bob generates his own random purple lines and adds them to the green lines to make his picture.

In Alice and Bob’s art project, can we argue that the green lines summarize “all and only” the information shared across the two pictures? Not necessarily with very formal math, but enough to see why it must be true. If you want to try this as an exercise, pause here.

.

.

.

Here’s our argument: first, the two pictures are independent (in the formal probability sense) conditional on the green lines. So the rest of Alice's picture tells us nothing more about Bob’s, once we have the green lines; in that sense, the green lines summarize all the information about Alice’s picture relevant to Bob’s (or vice-versa).

Second, notice that the green lines are included in their entirety in Bob’s picture. So the green lines include no “extra” information, nothing irrelevant to Bob’s picture. In that sense, the green lines summarize only the information about Alice’s picture relevant to Bob’s. Similarly, since the green lines are included in their entirety in Alice’s picture, they summarize only the information about Bob’s picture which is relevant to Alice’s.

Put those two together, and the green lines summarize all and only the information shared across the two pictures. In that sense, they are a “natural” summary of shared information - a natural latent.

Generalization

Looking back at the previous section, we can back out two intuitive criteria which suffice for the argument:

• Mediation ^[1]: Our candidate natural latent must mediate between parts of the system - formally, the parts must be independent given the latent.
• Redundancy ^[2]: Our candidate natural latent must be “included entirely” in each part of the system - i.e. the candidate’s value can be backed out from any individual part.

If those two criteria both hold, then the candidate summarizes “all and only” the information shared across the parts; it’s a natural latent.

Let’s walk through those criteria for the idealish gas and the dogs.

The isolated idealish gas is simple: the “parts'' are the low-level state of the gas at two (or more) sufficiently separated times.

• Mediation: Chaos ensures that the energy mediates between those states; their low-level details are all independent controlling for the energy.
• Redundancy: Since energy is conserved, it can be calculated from the state at any single time.

So, we have a natural latent.

The dogs example is trickier.

Dogs

We want to talk about a summary of “all and only” the information shared across dogs, separate from their individual details.

As a first pass, we might take the “parts” of our system to be individual dogs. But then we run into trouble: we might not be able to back out a summary of general properties of dogs from any one dog. For instance, any given dog might have three legs, even though the general statistical property of dogs is to usually have four.

So instead, we take the “parts” to be large nonoverlapping sets of individual dogs, without any particular sorting by breed or the like. Just take 1000 or 10000 randomly-selected dogs in the first set, another random 1000 or 10000 in the second set, etc. So long as each set is large and diverse enough to get a decent estimate of the general statistical properties of dogs, we can satisfy the redundancy condition. And so long as the dogs in different sets are independent given those general properties, we also satisfy the mediation condition. So, the general statistical properties of dogs would then be a natural latent between those random subsets of dogs.

Note that we’re relying on approximation here: those subsets definitely won’t give us very exact estimates of the general properties of dogs, and the dogs might not be exactly independent given a summary of those general properties. And that’s fine; the interesting properties of natural latents are robust to approximation. So long as the conditions are approximately satisfied, the latent captures approximately all and only the shared information.

Note also that this “take a random subset and statistically average or whatever” business is… not the most elegant. The mathematical formulation of natural latents is cleaner: rather than requiring that the latent be redundantly represented in every part of the system, we instead require that the latent can be reconstructed while ignoring any one part. So in the context of the dogs example: the general properties of dogs are not sensitive to any one dog. That’s a weaker and cleaner redundancy criterion, but it makes arguments/proofs less intuitive and quantitatively weaker, which is why we’re using a stronger redundancy criterion here. (We usually call natural latents under the stronger redundancy condition “strong natural latents”.) Point is: this can be made somewhat more elegant, at a cost of being less intuitive/elegant in other ways.

Why Are Natural Latents Useful?

Now that we have a few examples under our belt, let’s talk about two of the simplest stories of why natural latents are useful. (These aren’t the only stories of why natural latents are useful, but they’re the simplest and we expect that they underlie many others.)

Minimal Relevant Information

Recall our opening example, in which Carol watches Alice draw her purple and green lines, send the green lines to Bob, and Bob add his purple lines to them. After seeing Alice draw her picture, Carol wants to make predictions about Bob’s picture. We said that it seems natural for Carol to just remember where all the green lines are, and forget about the purple lines, since the green lines summarize “all and only” the information relevant to Bob’s picture.

That means that the green lines are the smallest summary of Alice’s picture which still keeps around everything relevant to Bob’s picture. After all, any other summary which keeps around everything relevant to Bob’s picture must at-minimum contain enough information to fully reconstruct the green lines.

If we wanted mathematical rigor, we could invoke something like the Gooder Regulator Theorem at this point. Walking through the conceptual story behind that theorem: we suppose that Carol is in a complicated world where she doesn’t know in advance which particular things about Bob’s picture she’ll need to predict, so she wants to keep around any information which might be relevant. But also, her mind is not infinite, she cannot remember everything, so she keeps around only the minimum summary of everything potentially relevant. Thus, the green lines.

More generally, this is one story for why real agents would want to track natural latents. Real agents don’t know what exact stuff they’ll need to predict later (because the world is complicated), but they also have limited memory, so insofar as there exists a minimal summary of everything which might be relevant, they likely track that.

So that’s one story.

Note, however, that a Gooder Regulator-style story fails to account for the usefulness of one of the examples we’ve already seen: dogs. In the dogs example, the “general statistical properties of dogs” are both too big to track in a human’s memory (e.g. the consensus dog genome alone is ~1.2 gigabytes) and not easy to figure out just from only surface-level observations of dogs (genome sequencing is nontrivial). Yet even when we don’t know the value of the natural latent (including e.g. the consensus genome), it’s apparently still useful to use that natural latent to structure our internal world-models - i.e. model dogs as though most of them are basically-independent given some general statistical properties (not all of which we know, but which can be approximately figured out from moderate-sized random samples of dogs). Why is that useful? Why do we humans apparently structure our thoughts that way? Those are open research questions; we don’t yet have all the mathematical answers to them, though we have some intuitions. And we do have one more story.

Maximal Robust Information

One of the basic arguments for natural abstraction being “a thing” is the language problem: the number of functions from a 1 megabyte image to a yes/no answer to “does this image contain a dog?” is 2^(2^8000000). To specify an arbitrary such function would therefore require ~2^8000000 examples. Allowing for lots of ambiguous edge-cases would change the number but not the qualitative conclusion: brute-force learning a function requires a ridiculously exponentially massive number of labeled examples. Yet humans, in practice, are able to usually basically figure out what other people mean by “dog” from something like one to five labeled examples - i.e. one to five instances of somebody pointing at a dog and saying “dog”. Language is able to work at all.

This tells us that “nearly all the work” of figuring out what “dogs” are must come, not from labeled examples, but from unsupervised learning: humans looking at the world and noticing statistical patterns which other humans also notice. We then get around the 2^8000000 problem by having only a relatively very very small set of candidate “things” to which words might be attached.

This basic picture puts some major constraints on the candidate “things”, even at this very rough level of characterization. Notably: in order for this whole language business to work, humans have to look at the world and notice statistical patterns which other humans also notice - despite those other humans not sharing exactly the same observations. You and I mostly agree on what “dogs” refers to, despite not seeing the same dogs.

… and that starts to sound like the redundancy condition for natural latents.

Let’s say we want candidate word-referents (like e.g. dogs) to be variables which can be estimated from any typical subset of some large set of stuff in the world, in order to handle the “humans which observe different subsets must nonetheless basically agree on what they’re talking about” problem. So we want them to satisfy the redundancy condition.

Now let’s get a bit stronger: let’s say we want candidate word-referents to be as informative as possible, subject to the redundancy condition. In other words, we want candidate word-referents to include all the information which can be deduced from a typical subset of the stuff in question - e.g. all the information which can be deduced from a typical subset of dogs.

Well, that means our word-referents will be natural latents, insofar as natural latents exist at all over the stuff in question.

There’s still a lot of sketchiness in this story! Perhaps most alarmingly, we’re defining the referent of “dogs” in terms of information deductible from typical subsets of dogs? Seems circular at first glance. That circularity will be resolved when we talk about clustering (below). For now, the main point is that the requirement of word-referent deductibility from typical subsets hand-wavily implies the redundancy condition, and the most informative variables satisfying redundancy are natural latents (when natural latents exist at all).

More Examples

Now that we’ve covered the basics, it’s time to go through a bunch of examples. These are chosen to illustrate common patterns, as well as common subtleties.

There’s a lot of examples here. For an 80/20 of the most important concepts, read the Coin Flip examples, the Gas (Over Space) example, and the “Clusters In Thingspace” example.

Toy Probability Examples

Anti-Example: Three Flips Of A Biased Coin

Natural latents don’t always exist over some given parts of a system. For example, suppose the “parts” are three different flips of the same coin, which has some unknown bias.

The obvious candidate for a natural latent would be the bias, since all the flips are independent conditional on the bias. So the mediation condition is satisfied. But the redundancy condition is not satisfied: a single flip (or even two) is not enough to figure out the bias of the coin.

So the obvious candidate doesn’t work. Can we rule out all candidate natural latents?

Well, the redundancy condition says that we must be able to back out the value of any natural latent from any one flip - e.g. the first coin flip. And that flip is just one bit. So any natural latent in this system would have to be just one bit (to within approximation, i.e. it probably takes one of two values), and that value must be (approximately) determined by the first coin flip. And there’s only one way that could happen: the potential natural latent would have to be approximately informationally equivalent to the first coinflip.

But then we observe that the three flips are not approximately independent given the first flip, nor can the first flip be backed out from the other two, so anything approximately informationally equivalent to the first flip is not a natural latent. Therefore, no natural latent exists in the three-flips system.

1000 Flips Of A Biased Coin

Now we have more flips. Let’s divide them into two sets: the first 500 flips, and the second 500.

The bias of the coin is still the obvious candidate natural latent, since the two sets of flips are independent given the bias. But now, we can estimate the bias (to reasonable precision with high probability) from either of the two sets, so the redundancy condition is satisfied. The bias is therefore a natural latent over the two sets of coin flips.

Note that we made a similar move here to the dogs example earlier: rather than considering each coin flip as one “part” of the system, we viewed whole sets of coin flips as the “parts”. As with the dogs example, this is somewhat inelegant and we could avoid it by using a weaker redundancy condition (as done in Natural Latents: The Math), at the cost of weaker quantitative bounds and somewhat less intuitive arguments.

Ising Model

The Ising model is a standard toy model of magnets. We have a large 2D square grid, and at each point on the grid a tiny magnet is randomly pointed either up or down. These being magnets, they like to align with their neighbors: each neighbor pointed up makes it more likely that this magnet will point up, and vice-versa.

The main thing usually studied in Ising models is a phase change: when the magnets are only weakly correlated with their neighbors, far-apart magnets are approximately independent. But as the correlations increase, eventually most of the magnets “click into” a matching orientation, so that even very far apart magnets are probably pointed the same way.

So what are the natural latents here?

Well, at low correlation, far-apart parts of the system are approximately independent, so the “empty latent” is natural - i.e. we don’t need to condition on anything. At higher correlation, far-apart parts are independent given the “consensus direction” of the full system, and that consensus direction can be estimated precisely from any one large-enough chunk. So at higher correlations, the consensus direction is the natural latent.

Notably, when the system undergoes phase change, there’s not just a change in values of the natural latents (like e.g. whether the consensus direction is up or down), but also in what variables are natural latents (i.e. what function to compute from a chunk in order to estimate the natural latent). In our work, we think of this as the defining feature of “phase change”; it’s a useful definition which generalizes intuitively to other domains.

Another notable feature of this example: the natural latents are over sufficiently far apart chunks of the system, much like our ideal gas example earlier. We didn’t emphasize it earlier, but one common pattern is that most of the “parts” of a system are independent of most others, but there may be a few parts which interact. In this case, if we took the parts to be each individual magnet, then we’d see that each magnet interacts weakly with its neighbors (and neighbors-of-neighbors, etc, though the interaction quickly drops off). That would mean there’s no natural latent, for similar reasons to the three-coinflips example above. But, so long as there exists a way to carve up the system into “parts” which avoids those “local” interactions, we can establish a natural latent. And typically, as in this example, there are many carvings which result in approximately the same natural latent - e.g. many choices of sufficiently spatially-separated “chunks”.

A similar issue implicitly came up in the dogs example: closely related dogs may be correlated in ways not mediated by the “general properties of dogs”. But most dogs are approximately independent of most others, so we can pick subsets to establish a natural latent, and we expect that many choices will yield approximately the same natural latent.

Physics-Flavored Examples

Gas (Over Space)

We’ve already said that the energy of an isolated idealish gas is a natural latent over the low-level state of the gas at sufficiently separated times. But we can go further.

So long as the gas is near equilibrium, not only is the low-level state at different times independent given the energy, the low-level states of sufficiently spatially-separated chunks of the gas are also independent given the energy. And, again invoking near-equilibrium, that energy can be precisely estimated from any one spatially-separated chunk, because the gas has approximately-uniform temperature. So, energy is a natural latent over low-level state of spatially-separated chunks of gas, in addition to time-separated.

Note, as in the Ising model, that there are local interactions not mediated by the natural latent. As usual, we choose “parts” to avoid those, and expect that many choices of “parts” yield the same natural latent. This is usually the case, and we’re going to stop emphasizing it in most of the examples.

Non-Isolated Gas

Our previous example assumes the gas is isolated. What if instead the gas is in a piston interacting with the world in the usual ways: expanding/compressing, heating/cooling, gas flowing in and out? Well, if we keep the “near equilibrium” assumption, spatially-separated chunks of the gas are no longer independent: by looking at one little chunk, I can deduce the temperature, pressure and number-density (i.e. number of molecules per unit volume) for the other chunks. But I do expect that the spatially-separated chunks are approximately independent given temperature, pressure, and number density. Furthermore, I can precisely estimate the temperature, pressure and number density from any one chunk. So, the triple (temperature, pressure, number density) is a natural latent over spatially-separated chunks of the gas at a fixed time. Note that these are exactly the “state variables” in the ideal gas law.

One thing to emphasize in the non-isolated gas example: temperature, pressure and number density all vary over time. The temperature, pressure and number density at a given time are a natural latent only over spatially-separated chunks of the gas at that time. Different parts of the world, like e.g. chunks of gas at different times, have different natural latents over them.

Another thing to emphasize: according the ideal gas law, the temperature T, pressure P and number density $\frac{n}{V}$ are not independent; we can calculate any one from the other two and the ideal gas constant $R$:

$P=\frac{n}{V}RT$

… so in fact any one of the three tuples $(P,T)$, $(T,\frac{n}{V})$, or $(\frac{n}{V},P)$ is a natural latent. All three are informationally equivalent (i.e. from any one we can calculate the others), they just represent the same information differently. More generally: natural latents are typically only of interest “up to isomorphism” - anything which represents exactly the same information is effectively the same latent.

Gasses In Systems

$T$, $P$, and $\frac{n}{V}$ are importantly different from a natural latents perspective once we stick the piston in a larger system. For instance, we might arrange the piston to push against another piston of the same radius containing gas of different number density and temperature:

Near equilibrium, the two pistons have approximately the same pressure, but (potentially) different temperature and number density. And we can measure the pressure from either piston, so the pressure alone is natural across the two pistons, whereas the tuple $(P,T_1)$ (or some informationally-equivalent tuple) is a natural latent over spatially-separated chunks of gas within piston 1, and $(P,T_2)$ is natural over spatially-separated chunks of gas within piston 2.

Similarly, we could put two pistons in thermal contact, so that near equilibrium the temperature is natural across the two.

Rigid Bodies

An example of a different flavor: humans tend to recognize rigid bodies as “objects”. Can we formulate that in terms of natural latents?

Well, consider a wooden chair. The chair has lots of little parts, but they all move approximately together: if I know the position and orientation of any one part at a particular time (along with the chair’s geometry), then I know roughly the position and orientation of all the other parts at that time. And given the high-level position and orientation of the chair, I typically mentally model the smaller vibrations of little parts as roughly independent (though I might need to use a more accurate model for e.g. acoustics of the chair, or heat flow). So, under that mental model, the position and orientation are a natural latent over the positions and orientations of the chair’s parts.

There’s also a natural latent over time: I can observe the chair’s geometry from its components at any given time, and under a simple mental model the positions of all the chair’s components at sufficiently separated times are independent given the chair’s geometry.

In this view, what makes the chair a coherent “thing” to view as a single rigid-body object is, in some sense, that it has these two natural latents: a geometry natural over time, and position/orientation natural over space at any given time. Insofar as the assumptions behind those two natural latents break down, it becomes less and less useful to view the chair as a single rigid-body object.

Note that, in this example, we ignored things like acoustics or heat flow. One way to “more rigorously” ignore such components would be to say “position and orientation are a natural latent over the rigid body given acoustics and temperature”. More generally, it’s totally legit to have latents which are natural after we condition on some other “background” information. Of course, it then potentially matters what “background” information we’re conditioning on at any given time.

Phase Change

Returning to the gas in a piston: suppose we cool the gas until it solidifies. How does that change the natural latents?

Well, temperature and pressure are presumably no longer a natural latent; assuming the resulting solid is reasonably rigid, the position/orientation can be measured from each little piece, and there’s a conserved geometry, as in the previous section.

As in the Ising model, when the system undergoes phase change, there’s not just a change in values of the natural latents (like e.g. change in temperature or in pressure), but also in what variables are natural latents.

Other Examples

“Clusters In Thingspace”

First, some 101 background…

Wrong Novice: All real-world categories have weird edge cases: dogs with three legs, cars with the wheels removed, etc. It is therefore impossible to have precise definitions of words.

Less Wrong Novice: Words point to clusters in thingspace. Just because there are always individual points “on the edges” of the clusters, doesn’t mean there’s no precise characterization of the clusters themselves. The precise definition of “dogs” is not e.g. a cutting-plane in thingspace such that all the dogs are on one side and the non-dogs are on another side; rather, the precise definition of “dogs” is just the statistics which characterize the dog-cluster - i.e. the general properties of dogs.

Researcher: That seems intuitively sensible, but this “thingspace” is pretty handwavy. Different choices of space (i.e. the axes of “thingspace”) will yield very different clusters. So what’s the space in which we’re clustering, and how do we do that clustering?

Let’s first see what a natural-latents-centric frame has to say to that researcher, then talk about how natural latents relate to this "clusters in thingspace" picture more generally.

One important thing about dogs is that there are many different ways to tell something is a dog - many different subsets of “the dog” which I could look at, many different properties of the dog, etc. Face, fur, genome, bark, collar, etc. In other words, any individual dog satisfies a redundancy condition.

In the clustering picture, that redundancy condition tells us something about the axes of “thingspace”: there’s a lot of axes, and the clusters are reasonably robust to which axes we use. I could cluster based on genome, I could cluster based on properties like face/fur/bark/etc, and I’ll get a cluster of dogs either way.

Insofar as that’s true (and there are cases where it breaks down to some extent, though arguably those cases are the exceptions which prove the rule), the researcher’s question about choice of axes largely goes away. So long as we pick a typical wide variety of axes of thingspace, we expect the exact choices to not matter much - or, more precisely, we’ll specifically look for clusters such that the choice of axes doesn’t matter much.

With that in mind, we’re now ready to return to the “circularity of ‘dogs’” problem from earlier.

Recall the problem: roughly speaking, we’d like to say that “dogs” are defined in terms of some general statistical properties of dogs, i.e. the statistical parameters of the dog-cluster. But that’s circular, because which-things-count-as-”dogs”-to-what-degree depends on the cluster parameters, and the cluster parameters depend on which-things-count-as-”dogs”-to-what-degree.

With the clustering picture in mind, we see that this is… just the ordinary circularity of clustering problems. One of the most standard clustering algorithms is basically:

• Estimate assignments of points to clusters, given current estimate of cluster parameters
• Estimate cluster parameters, given current estimate of points to clusters
• Repeat until convergence

And yeah, that is somewhat circular. There are sometimes multiple “solutions” - i.e. multiple fixed points of the iteration. But in practice, there’s a relatively small and typically discrete set of convergent “solutions”, and most clusters match most of the time across runs. And that’s what we expect from natural abstraction in general: clearly not all agents match in all their chosen abstractions, but there’s a relatively small and discrete set of possibilities, and they mostly match most of the time. It’s not like choosing arbitrary points on a continuum, but rather like choosing which books to take off of a shelf, where the environment (not the agent) is the librarian.

That two-repeated-step clustering algorithm also suggests that there are two places where natural latents enter this sort of clustering picture. First: the assignment of a point to a cluster (e.g. “is this a dog?”) is a natural latent over the “coordinates in thingspace” of the point - i.e. we can tell whether this is a dog by looking at genome, face/fur/etc, or many other things, and lots of those properties should become independent once we know it’s a dog. Second: the cluster parameters should be natural latents over the points in a cluster - i.e. the general properties of dogs should be a natural latent over dogs, as we discussed earlier.

Social Constructs: Laws

When I talk about natural abstraction, people often bring up social constructs, with laws being a central example. Laws are entirely “chosen by humans” in some sense; humans could all change what they consider to be laws, and that would change the laws. Contrast that with a vibe-y picture of natural abstraction, in which the natural abstractions are supposed to be “determined by the environment”, in some sense, such that a wide variety of other minds arrive at the same abstractions.

In this example, we’ll walk through a standard response explaining how social constructs are natural abstractions in a nontrivial sense, and see how that response applies to natural latents as a specific formulation of natural abstraction.

First, a background concept. For visualization, let’s picture a clustering problem:

One of these clusters consists of minivans, and the other consists of pickup trucks. Well, if people want to, they can totally just go build new kinds of vehicles which fall in other parts of the space:

… and if they build enough new vehicles, it can totally change what clusters are present:

Two points here:

• People can totally change what stuff is in the world, in such a way that the “natural abstractions” change. This extends to natural latents.
• … but that requires going out and changing stuff in the world; it’s not just a matter of me deciding to assign different clusters to the same data.

So even if the clusters can be changed by humans by changing the world, that doesn’t mean that choice of clusters is a totally arbitrary choice of how to view the same data.

Now let’s talk about laws.

First: insofar as lots of different people agree on what the laws are, lots of different police/courts enforce the laws mostly-consistently, etc, that means the laws are redundant over all those people/police/courts. So, just from the redundancy property, we know that any natural latent over subsets of those people must include the laws. (We’re ignoring the independence property here, because when we get to fairly abstract things like laws independence usually involves conditioning on a whole stack of lower-level things, which isn’t central to the point here.)

Insofar as alien minds indeed structure their world-models around natural latents, and those alien minds observe and model all these different people using the same laws, those aliens will therefore include those laws in any natural latent over the people (assuming such a natural latent exists).

As in our clustering visualizations, the humans could collectively decide to adopt different laws. The alien could even try to influence the humans to adopt different laws. But crucially, this requires going out and changing something about all those humans out in the world; it’s not just a matter of the alien deciding to assign different concepts to the same data. Likewise for any individual human: I cannot change the laws just by viewing my own data differently; changing the laws requires going out and changing the behavior of lots of other humans.

Takeaways

Natural latents are pretty handy. If a variable is a natural latent over some parts of a system, then I know it’s the smallest summary of everything about one part relevant to the others, and I know it’s informationally the largest thing which I can learn from a typical subset of the chunks. That makes such latents natural for agents to structure their cognition and language around.

A quick intuitive check for whether something is a natural latent over some parts of a system consists of two questions:

• Are the parts (approximately) independent given the candidate natural latent?
• Can the candidate natural latent be estimated to reasonable precision from any one part, or any typical subset of the parts?

1. ^{^}

   We also sometimes call the mediation condition the “independence” condition.

2. ^{^}

   Natural Latents: The Math called the redundancy condition the “insensitivity” condition instead; we also sometimes call it the “invariance” condition.