Mechanistic Transparency for Machine Learning

by DanielFilan3 min read11th Jul 20189 comments

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Transparency / Interpretability (ML & AI)AI
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Crossposted from the AI Alignment Forum. May contain more technical jargon than usual.

Cross-posted on my blog.

Lately I've been trying to come up with a thread of AI alignment research that (a) I can concretely see how it significantly contributes to actually building aligned AI and (b) seems like something that I could actually make progress on. After some thinking and narrowing down possibilities, I've come up with one -- basically, a particular angle on machine learning transparency research.

The angle that I'm interested in is what I'll call mechanistic transparency. This roughly means developing tools that take a neural network designed to do well on some task, and outputting something like pseudocode for what algorithm the neural network implements that could be read and understood by developers of AI systems, without having to actually run the system. This pseudocode might use high-level primitives like 'sort' or 'argmax' or 'detect cats', that should themselves be able to be reduced to pseudocode of a similar type, until eventually it is ideally reduced to a very small part of the original neural network, small enough that one could understand its functional behaviour with pen and paper within an hour. These tools might also slightly modify the network to make it more amenable to this analysis in such a way that the modified network performs approximately as well as the original network.

There are a few properties that this pseudocode must satisfy. Firstly, it must be faithful to the network that is explained, such that if one substitutes in the pseudocode for each high-level primitive recursively, the result should be the original neural network, or a network close enough to the original that the differences are irrelevant (although just in case, the reconstructed network that is exactly explained should presumably be the one deployed). Secondly, the high-level primitives must be somewhat understandable: the pseudocode for a 256-layer neural network for image classification should not be output = f2(f1(input)) where f1 is the action of the first 128 layers and f2 is the action of the next 128 layers, but rather break down into edge detectors being used to find floppy ears and spheres and textures, and those being combined in reasonable ways to form judgements of what the image depicts. The high-level primitives should be as human-understandable as possible, ideally 'carving the computation at the joints' by representing any independent sub-computations or repeated applications of the same function (so, for instance, if a convolutional network is represented as if it were fully connected, these tools should be able to recover convolutional structure). Finally, the high-level primitives in the pseudocode should ideally be understandable enough to be modularised and used in different places for the same function.

This agenda nicely relates to some existing work in machine learning. For instance, I think that there are strong synergies with research on compression of neural networks. This is partially due to background models about compression being related to understanding (see the ideas in common between Kolmogorov complexity, MDL, Solomonoff induction, and Martin-Löf randomness), and partially due to object-level details about this research. For example, sparsification seems related to increased modularity, which should make it easier to write understandable pseudocode. Another example is the efficacy of weight quantisation, which means that the least significant bits of the weights aren't very important, indicating that the relations between the high-level primitives should be modular in an understandable way and not have crucial details depend on some of the least significant bits of the output.

The Distill post on the building blocks of interpretability includes some other examples of work that I feel is relevant. For instance, work on using matrix factorisation to group neurons seems very related to constructing high-level primitives, and work on neuron visualisation should help with understanding the high-level primitives if their output corresponds to a subset of neurons in the original network.

I'm excited about this agenda because I see it as giving the developers of AI systems tools to detect and correct properties of their AI systems that they see as undesirable, without having to deploy the system in a test environment that they must laboriously ensure is adequately sandboxed. You could imagine developers checking if their systems conform to theories of aligned AI, or detecting any 'deceive the human' subroutine that might exist. I see this as fairly robustly useful, being helpful in most stories of how one would build an aligned AI. The exception is if AGI is built without things which look like modern machine learning algorithms, which I see as unlikely, and at any rate hope that lessons transfer to the methods which are used.

I also believe that this line of research has a shot at working for systems which act in the world. It seems hard for me to describe how I detect laptops given visual informations, but given visual primitives like 'there's a laptop there', it seems much easier for me to describe how I play tetris or even go. As such, I would expect tools developed in this way to illuminate the strategy followed by tetris-playing DQNs by referring to high-level primitives like 'locate T tetronimo', that themselves would have to be understood using neuron visualisation techniques.

Visual primitives are probably not the only things that would be hard to fully understand using the pseudocode technique. In cases where humans evade oversight by other humans, I assert that it is often not due to consequentialist reasoning, but rather due to avoiding things which are frustrating or irritating, where frustration/irritation is hard to introspect on but seems to reliably steer away from oversight in cases where that oversight would be negative. A possible reason that this frustration/irritation is hard to introspect upon is that it is complicated and hard to decompose cleanly, like our object recognition systems are. Similarly, you could imagine that one high-level primitive that guides the AI system's behaviour is hard to decompose and needs techniques like neuron visualisation to understand. However, at least the mechanistic decomposition allowed us to locate this subsystem and determine how it is used in the network, guiding the tests we perform on it. Furthermore, in the case of humans, it's quite possible that our frustration/irritation is hard to introspect upon not because it's hard to understand, but rather because it's strategically better to not be able to introspect upon it (see the ideas in the book The Elephant in the Brain), suggesting that this problem might be less severe than it seems.

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In programming, it's often easier to write new code from scratch than to try to understand someone else's code, especially if the other person's code is optimized for something other than human-understandability. See here for an example, where I wrote:

Many of the algorithms and tables used here came from the deflate implementation by Jean-loup Gailly, which was included in Crypto++ 4.0 and earlier. I completely rewrote it in order to fix a bug that I could not figure out. This code is less clever, but hopefully more understandable and maintainable.

Since human-understandability is costly to evaluate (and hence to train), and also costly in terms of causing lower performance on other metrics (note that code that I wrote to be more understandable is significantly slower than the original code), I have strong doubts about this line of research.

My guess is that if you took a human-level AGI that was the result of something like deep learning optimizing only for capability (and not understandability), and tried to interpret it as pseudocode, you'll end up with so many modules with so many interactions between them that no human or team of humans could understand it. In other words, you'll end up with spaghetti code written by a superintelligence (meaning the training process).

If you instead tried to optimize for both capability and understandability at the same time, you have a much harder ML problem on your hands, maybe even an impossible one.

Perhaps if an AGI is built out of modules that are separately trained, instead of being trained end-to-end, you could use this idea on some of the smaller modules that are especially important to safety. I'm curious if that's the kind of plan you have in mind, or if you're more ambitious about this approach.

This response is rather late, but basically my hope is that it's possible to optimise for understandability by regularising for some relatively simple quantity that induces understandability.

Perhaps if an AGI is built out of modules that are separately trained, instead of being trained end-to-end, you could use this idea on some of the smaller modules that are especially important to safety. I'm curious if that's the kind of plan you have in mind, or if you're more ambitious about this approach.

I'm more ambitious, and fear that that might not work: either you train a bunch of 'small' things that do very concrete tasks, and aren't quite sure how to combine them to create AGI (or you have to combine a huge number of them and hope that errors don't cascade), or you train a few large ones that do big, complicated tasks that themselves are hard to interpret. That being said, the first branch would satisfy my desiderata for the approach, and I'd hope some people are working on it.

I see two major challenges (one of which leans heavily on progress in linguistics). I can see there being mathematical theory to guide candidate model decompositions (Challenge 1), but I imagine that linking up a potential model decomposition to a theory of 'semantic interpretability' (Challenge 2) is equally hard, if not harder.

Any ideas on how you plan to address Challenge 2? Maybe the most robust approach would involve active learning of the pseudocode, where a human guides the algorithm in its decomposition and labeling of each abstract computation.

Thoughts on challenge 2:

  • 'Smaller' functions will probably be more human-interpretable, just because they do less, are easy to analyse, and have less weird stuff going on. I think that this implies that as you 'double-click' on more high-level primitives, they get more and more interpretable.
  • It's plausible to me that there's some mathematical theory of how to get things that are human-interpretable enough for our purposes.
  • It's also plausible to me that by trying enough things, you find a method that seems sort of human-interpretable, see what properties it actually has, and check if you can use those.
  • There might be synergies with interpretability techniques like neuron visualisation that give you a sense of the input-output behaviour without telling you much about the internal mechanisms.
  • If a neural network is well-trained, it's easier to visualise what each neuron does, because intuitively they need to do sensible things for the outputs to be sensible. You could hope that a similar property for high-level primitives obtains if those primitives are constructed sensibly out of neurons.

"Programmatically Interpretable Reinforcement Learning" (Verma et al.) seems related. It would be great to see modular, understandable glosses of neural networks.

+1

I think I've already mentioned these papers to you, but just in case I haven't, to add on to the one Nisan suggested:

Verifiable Reinforcement Learning via Policy Extraction

Towards Mixed Optimization for Reinforcement Learning with Program Synthesis

No substantive comment to make, but I think this is a great direction of research. I'm reminded of the chilling bit in Crystal Nights where a scientist responsible for watching a simulated civilization says "We're losing track of the language".

I'm not sure if this will be helpful or if you've already explored this connection, but the field of abstract interpretation tries to understand the semantics of a computer program without fully executing it. The theme of "trying to understand what a program will do by just examining its source code" is also present in program analysis. If we can understand neural networks as typed functional programs maybe there's something worth thinking about here.

I'm curious how a program could take another program and figure out its purpose is to detect cats.