Zach Furman
Repeating a question I asked Jesse earlier, since others might be interested in the answer: how come we tend to hear more about PAC bounds than MAC bounds?
Learning coefficient estimation: the details
Note that in the SLT setting, “brains” or “neural networks” are not the sorts of things that can be singular (or really, have a certain ) on their own—instead they’re singular for certain distributions of data.
This is a good point I often see neglected. Though there’s some sense in which a model can “be singular” independent of data: if the parameter-to-function map is not locally injective. Then, if a distribution minimizes the loss, the preimage of in parameter space can have non-trivial geometry.
These are called “degeneracies,” and they can be understood for a particular model without talking about data. Though the actual that minimizes the loss is determined by data, so it’s sort of like the “menu” of degeneracies are data-independent, and the data “selects one off the menu.” Degeneracies imply singularities, but not necessarily vice-versa, so they aren’t everything. But we do think that degeneracies will be fairly important in practice.
A possible counterpoint, that you are mostly advocating for awareness as opssosed to specific points is null, since pretty much everyone is aware of the problem now—both society as a whole, policymakers in particular, and people in AI research and alignment.
I think this specific point is false, especially outside of tech circles. My experience has been that while people are concerned about AI in general, and very open to X-risk when they hear about it, there is zero awareness of X-risk beyond popular fiction. It’s possible that my sample isn’t representative here, but I would expect that to swing in the other direction, given that the folks I interact with are often well-educated New-York-Times-reading types, who are going to be more informed than average.
Even among those aware, there’s also a difference between far-mode “awareness” in the sense of X-risk as some far away academic problem, and near-mode “awareness” in the sense of “oh shit, maybe this could actually impact me.” Hearing a bunch of academic arguments, but never seeing anybody actually getting fired up or protesting, will implicitly cause people to put X-risk in the first bucket. Because if they personally believed it to be big a near-term risk, they’d certainly be angry and protesting, and if other people aren’t, that’s a signal other people don’t really take it seriously. People sense a missing mood here and update on it.
In the cybersecurity analogy, it seems like there are two distinct scenarios being conflated here:
1) Person A says to Person B, “I think your software has X vulnerability in it.” Person B says, “This is a highly specific scenario, and I suspect you don’t have enough evidence to come to that conclusion. In a world where X vulnerability exists, you should be able to come up with a proof-of-concept, so do that and come back to me.”
2) Person B says to Person A, “Given XYZ reasoning, my software almost certainly has no critical vulnerabilities of any kind. I’m so confident, I give it a 99.99999%+ chance.” Person A says, “I can’t specify the exact vulnerability your software might have without it in front of me, but I’m fairly sure this confidence is unwarranted. In general it’s easy to underestimate how your security story can fail under adversarial pressure. If you want, I could name X hypothetical vulnerability, but this isn’t because I think X will actually be the vulnerability, I’m just trying to be illustrative.”
Story 1 seems to be the case where “POC or GTFO” is justified. Story 2 seems to be the case where “security mindset” is justified.
It’s very different to suppose a particular vulnerability exists (not just as an example, but as the scenario that will happen), than it is to suppose that some vulnerability exists. Of course in practice someone simply saying “your code probably has vulnerabilities,” while true, isn’t very helpful, so you may still want to say “POC or GTFO”—but this isn’t because you think they’re wrong, it’s because they haven’t given you any new information.
Curious what others have to say, but it seems to me like this post is more analogous to story 2 than story 1.
I wish I had a more short-form reference here, but for anyone who wants to learn more about this, Rocket Propulsion Elements is the gold standard intro textbook. We used in my university rocketry group, and it’s a common reference to see in industry. Fairly well written, and you should only need to know high school physics and calculus.
Obviously this is all speculation but maybe I’m saying that the universal approximation theorem implies that neural architectures are fractal in space of all distributtions (or some restricted subset thereof)?
Oh I actually don’t think this is speculation, if (big if) you satisfy the conditions for universal approximation then this is just true (specifically that the image of is dense in function space). Like, for example, you can state Stone-Weierstrass as: for a Hausdorff space X, and the continuous functions under the sup norm , the Banach subalgebra of polynomials is dense in . In practice you’d only have a finite-dimensional subset of the polynomials, so this obviously can’t hold exactly, but as you increase the size of the polynomials, they’ll be more space-filling and the error bound will decrease.Curious what’s your beef with universal approximation? Stone-weierstrass isn’t quantitative—is that the reason?
The problem is that the dimension of required to achieve a given error bound grows exponentially with the dimension of your underlying space . For instance, if you assume that weights depend continuously on the target function, -approximating all functions on with Sobolev norm provably takes at least parameters (DeVore et al.). This is a lower bound.
So for any realistic universal approximation is basically useless—the number of parameters required is enormous. Which makes sense because approximation by basis functions is basically the continuous version of a lookup table.
Because neural networks actually work in practice, without requiring exponentially many parameters, this also tells you that the space of realistic target functions can’t just be some generic function space (even with smoothness conditions), it has to have some non-generic properties to escape the lower bound.
Sorry, I realized that you’re mostly talking about the space of true distributions and I was mainly talking about the “data manifold” (related to the structure of the map for fixed ). You can disregard most of that.
Though, even in the case where we’re talking about the space of true distributions, I’m still not convinced that the image of under needs to be fractal. Like, a space-filling assumption sounds to me like basically a universal approximation argument—you’re assuming that the image of densely (or almost densely) fills the space of all probability distributions of a given dimension. But of course we know that universal approximation is problematic and can’t explain what neural nets are actually doing for realistic data.
Very interesting, glad to see this written up! Not sure I totally agree that it’s necessary for to be a fractal? But I do think you’re onto something.
In particular you say that “there are points in the larger dimensional space that are very (even arbitrarily) far from ,” but in the case of GPT-4 the input space is discrete, and even in the case of e.g. vision models the input space is compact. So the distance must be bounded.Plus if you e.g. sample a random image, you’ll find there’s usually a finite distance you need to travel in the input space (in L1, L2, etc) until you get something that’s human interpretable (i.e. lies on the data manifold). So that would point against the data manifold being dense in the input space.
But there is something here, I think. The distance usually isn’t that large until you reach a human interpretable image, and it’s quite easy to perturb images slightly to have completely different interpretations (both to humans and ML systems). A fairly smooth data manifold wouldn’t do this. So my guess is that the data “manifold” is in fact not a manifold globally, but instead has many self-intersections and is singular. That would let it be close to large portions of input space without being literally dense in it. This also makes sense from an SLT perspective. And IIRC there’s some empirical evidence that the dimension of the data “manifold” is not globally constant.
if the distribution of intermediate neurons shifts so that Othello-board-state-detectors have a reasonably high probability of being instantiated
Yeah, this “if” was the part I was claiming permutation invariance causes problems for—that identically distributed neurons probably couldn’t express something as complicated as a board-state-detector. As soon as that’s true (plus assuming the board-state-detector is implemented linearly), agreed, you can recover it with a linear probe regardless of permutation-invariance.
This is a more reasonable objection(although actually, I’m not sure if independence does hold in the tensor programs framework—probably?)
I probably should’ve just gone with that one, since the independence barrier is the one I usually think about, and harder to get around (related to non-free-field theories, perturbation theory, etc).
My impression from reading through one of the tensor program papers a while back was that it still makes the IID assumption, but there could be some subtlety about that I missed.
The reason the Othello result is surprising to the NTK is that neurons implementing an “Othello board state detector” would be vanishingly rare in the initial distribution, and the NTK thinks that the neuron function distribution does not change during training.
Yeah, that’s probably the best way to explain why this is surprising from the NTK perspective. I was trying to include mean-field and tensor programs as well (where that explanation doesn’t work anymore).
As an example, imagine that our input space consisted of five pixels, and at initialization neurons were randomly sensitive to one of the pixels. You would easily be able to construct linear probes sensitive to individual pixels even though the distribution over neurons is invariant over all the pixels.
Yeah, this is a good point. What I meant to specify wasn’t that you can’t recover any permutation-sensitive data at all (trivially, you can recover data about the input), but that any learned structures must be invariant to neuron permutation. (Though I’m feeling sketchy about the details of this claim). For the case of NTK, this is sort of trivial, since (as you pointed out) it doesn’t really learn features anyway.
By the way, there are actually two separate problems that come from the IID assumption: the “independent” part, and the “identically-distributed” part. For space I only really mentioned the second one. But even if you deal with the identically distributed assumption, the independence assumption still causes problems.This prevents a lot of structure from being representable—for example, a layer where “at most two neurons are activated on any input from some set” can’t be represented with independently distributed neurons. More generally a lot of circuit-style constructions require this joint structure. IMO this is actually the more fundamental limitation, though takes longer to dig into.
I think the core surprising thing is the fact that the model learns a representation of the board state. The causal / linear probe parts are there to ensure that you’ve defined “learns a representation of the board state” correctly—otherwise the probe could just be computing the board state itself, without that knowledge being used in the original model.
This is surprising to some older theories like statistical learning, because the model is usually treated as effectively a black box function approximator. It’s also surprising to theories like NTK, mean-field, and tensor programs, because they view model activations as IID samples from a single-neuron probability distribution—but you can’t reconstruct the board state via a permutation-invariant linear probe. The question of “which neuron is which” actually matters, so this form of feature learning is beyond them. (Though there may be e.g. perturbative modifications to these theories to allow this in a limited way).
Yeah, that was what I was referring to. Maybe “algorithmic model” isn’t the most precise—what we know is that the NN has an internal model of the board state that’s causal (i.e. the NN actually uses it to make predictions, as verified by interventions). Theoretically it could just be forming this internal model via a big lookup table / function approximation, rather than via a more sophisticated algorithm. Though we’ve seen from modular addition work, transformer induction heads, etc that at least some of the time NNs learn genuine algorithms.
Agreed—that alone isn’t particularly much, just one of the easier things to express succinctly. (Though the fact that this predates deep learning does seem significant to me. And the fact that SLT can delineate precisely where statistical learning theory went wrong here seems important too.)
Another is that can explain phenomena like phase transitions, as observed in e.g. toy models of superposition, at a quantitative level. There’s also been a substantial chunk of non-SLT ML literature that has independently rediscovered small pieces of SLT, like failures of information geometry, importance of parameter degeneracies, etc. More speculatively, but what excites me most, is that empirical phenomena like grokking, mode connectivity, and circuits seem to intuitively fit in SLT nicely, though this hasn’t been demonstrated rigorously yet.
Yeah, I can expand on that—this is obviously going be fairly opinionated, but there are a few things I’m excited about in this direction.
The first thing that comes to mind here is singular learning theory. I think all of my thoughts on DL theory are fairly strongly influenced by it at this point. It definitely doesn’t have all the answers at the moment, but it’s the single largest theory I’ve found that makes deep learning phenomena substantially “less surprising” (bonus points for these ideas preceding deep learning). For instance, one of the first things that SLT tells you is that the effective parameter count (RLCT) of your model can vary depending on the training distribution, allowing it to basically do internal model selection—the absence of bias-variance tradeoff, and the success of overparameterized models, aren’t surprising when you internalize this. The “connection to real world structure” aspect hasn’t been fully developed here, but it seems heavily suggested by the framework, in multiple ways—for instance, hierarchical statistical models are naturally singular statistical models, and the hierarchical structure is reflected in the singularities. (See also Tom Waring’s thesis).
Outside of SLT, there’s a few other areas I’m excited about—I’ll highlight just one. You mentioned Lin, Tegmark, and Rolnick—the broader literature on depth separations and the curse of dimensionality seems quite important. The approximation abilities of NNs are usually glossed over with universal approximation arguments, but this can’t be enough—for generic Lipschitz functions, universal approximation takes exponentially many parameters in the input dimension (this is a provable lower bound). So there has to be something special about the functions we care about in the real world. See this section of my post for more information. I’d highlight Poggio et al. here, which is the paper in the literature closest to my current view on this.
This isn’t a complete list, even of theoretical areas that I think could specifically help address the “real world structure” connection, but these are the two I’d feel bad not mentioning. This doesn’t include some of the more empirical findings in science of DL that I think are relevant, like simplicity bias, mode connectivity, grokking, etc. Or work outside DL that could be helpful to draw on, like Boolean circuit complexity, algorithmic information theory, natural abstractions, etc.
I can’t speak for Richard, but I think I have a similar issue with NTK and adjacent theory as it currently stands (beyond the usual issues). I’m significantly more confident in a theory of deep learning if it cleanly and consistently explains (or better yet, predicts) unexpected empirical phenomena. The one that sticks out most prominently in my mind, that we see constantly in interpretability, is this strange correspondence between the algorithmic “structure” we find in trained models (both ML and biological!) and “structure” in the data generating process.
That training on Othello move sequences gets you an algorithmic model of the game itself is surprising from most current theoretical perspectives! So in that sense I might be suspicious of a theory of deep learning that fails to “connect our understanding of neural networks to our understanding of the real world”. This is the single most striking thing to come out of interpretability, in my opinion, and I’m worried about a “deep learning theory of everything” if it doesn’t address this head on.
That said, NTK doesn’t promise to be a theory of everything, so I don’t mean to hold it to an unreasonable standard. It does what it says on the tin! I just don’t think it’s explained a lot of the remaining questions I have. I don’t think we’re in a situation where “we can explain 80% of a given model’s behavior with the NTK” or similar. And this is relevant for e.g. studying inductive biases, as you mentioned.
But I strong upvoted your comment, because I do think deep learning theory can fill this gap—I’m personally trying to work in this area. There are some tractable-looking directions here, and people shouldn’t neglect them!
Someone with better SLT knowledge might want to correct this, but more specifically:
Studying the “volume scaling” of near-min-loss parameters, as beren does here, is really core to SLT. The rate of change of this volume as you change your epsilon loss tolerance is called the “density of states” (DOS) function, and much of SLT basically boils down to an asymptotic analysis of this function. It also relates the terms in the asymptotic expansion to things you care about, like generalization performance.
You might wonder why SLT needs so much heavy machinery, since this sounds so simple—and it’s basically because SLT can handle the case where the eigenvalues of the Hessian are zero, and the usual formula breaks down. This is actually important in practice, since IIRC real models often have around 90% zero eigenvalues in their Hessian. It also leads to substantially different theory—for instance the “effective number of parameters” (RLCT) can vary depending on the dataset.
If anyone wants an interactive visual for the correlation upper and lower bounds, I made one for myself here: https://www.math3d.org/chXa4xZrC.
The x, y, and z axes are the correlations between X and Y, and Y and Z, and X and Z respectively. Everything inside the blue surface is possible. There are also some sliders to help visualize level sets.
Yep, pre-LN transformers avoid the vanishing gradient problem.
Haven’t checked this myself, but the phenomenon seems to be fairly clean? See figure 3.b in the paper I linked, or figure 1 in this paper.
I actually wouldn’t think of vanishing/exploding gradients as a pathological training problem but a more general phenomenon about any dynamical system. Some dynamical systems (e.g. the sigmoid map) fall into equilibria over time, getting exponentially close to one. Other dynamical systems (e.g. the logistic map) become chaotic, and similar trajectories diverge exponentially over time. If you check, you’ll find the first kind leads to vanishing gradients (at each iteration of the map), and the second to exploding ones. This a forward pass perspective on the problem—the usual perspective on the problem considers only implications for the backward pass, since that’s where the problem usually shows up.
Notice above that the system with exponential decay in the forward pass had vanishing gradients (growing gradient norms) in the backward pass—the relationship is inverse. If you start with toy single-neuron networks, you can prove this to yourself pretty easily.
The predictions here are still complicated by a few facts—first, exponential divergence/convergence of trajectories doesn’t necessarily imply exponentially growing/shrinking norms. Second, the layer norm complicates things, confining some dynamics to a hypersphere (modulo the zero-mean part). Haven’t fully worked out the problem for myself yet, but still think there’s a relationship here.
It’s worth noting that Jesse is mostly following the traditional “approximation, generalization, optimization” error decomposition from learning theory here—where “generalization” specifically refers to finite-sample generalization (gap between train/test loss), rather than something like OOD generalization. So e.g. a failure of transformers to solve recursive problems would be a failure of approximation, rather than a failure of generalization. Unless I misunderstood you?