# What's General-Purpose Search, And Why Might We Expect To See It In Trained ML Systems?

[Benito](https://www.lesswrong.com/users/benito) has an interesting job. Here’s some of the stuff he’s had to do over the past couple years:

*   build a prototype of an office
*   resolve neighbor complaints at a party
*   find housing for 13 people with 2 days notice
*   figure out an invite list for 100+ people for an office
*   deal with people emailing a funder trying to get him defunded
*   set moderation policies for LessWrong
*   write public explanations of grantmaking decisions
*   organize weekly online zoom events
*   ship books internationally by Christmas
*   moderate online debates
*   do April Fools' Jokes on Lesswrong
*   figure out which of 100s of applicants to do trial hires with

Quite a wide variety!

Benito illustrates an interesting feature of humans: you can give humans pretty arbitrary goals, pretty arbitrary jobs to do, pretty arbitrary problems to solve, and they'll go figure out how to do it. It seems like humans have some sort of “general-purpose problem-solving” capability.

Now, there’s more than one part of general-purpose problem solving. There’s efficient information-gathering and model-building and updating. There’s searching for promising plans. There’s execution (or, in the organizational context, operations). A general-purpose problem-solver needs general-purpose versions of all those. But for this post, I want to focus on the “searching for promising plans” part.

First things first: what is this “search” thing, anyway?

Babble And Prune Is Not The Only Search Method
----------------------------------------------

This whole post started out because I was talking about “search” (in the context of [an inner alignment strategy](https://www.lesswrong.com/posts/w4aeAFzSAguvqA5qu/how-to-go-from-interpretability-to-alignment-just-retarget)) and it turned out that people had radically different pictures of what the word “search” means. In particular, it turned out that a bunch of people pessimistic about the strategy were picturing some variant of [babble and prune](https://www.lesswrong.com/s/pC6DYFLPMTCbEwH8W): “babble” candidate solutions, then “prune” unpromising solutions, and hopefully iterate toward better and better solutions.

This is not really how humans search for promising plans. Consider, for example, a human planning a trip to the grocery store. Typical reasoning (mostly at the subconscious level) might involve steps like:

*   There’s a dozen different stores in different places, so I can probably find one nearby wherever I happen to be; I don’t need to worry about picking a location early in the planning process.
*   My calendar is tight, so I need to pick an open time. That restricts my options a lot, so I should worry about that early in the planning process.
    *   <go look at calendar>
*   Once I’ve picked an open time in my calendar, I should pick a grocery store nearby whatever I’m doing before/after that time.
*   … Oh, but I also need to go home immediately after, to put any frozen things in the freezer. So I should pick a time when I’ll be going home after, probably toward the end of the day.

Notice that this sort of reasoning mostly does *not* involve babbling and pruning entire plans. The human is thinking mostly at the level of constraints (and associated heuristics) which rule out broad swaths of plan-space. The calendar is a taut constraint, location is a slack constraint, so (heuristic) first find a convenient time and then pick whichever store is closest to wherever I’ll be before/after. The reasoning only deals with a few abstract plan-features (i.e. time, place) and ignores lots of details (i.e. exact route, space in the car’s trunk); more detail can be filled out later, so long as we’ve planned the “important” parts. And rather than “iterate” by looking at many plans, the search process mostly “iterates” by considering subproblems (like e.g. finding an open calendar slot) or adding lower-level constraints to a higher-level plan (like e.g. needing to get frozen goods home quickly).

So humans’ general-purpose internal search mostly doesn’t look like babble and prune in the classic sense. It’s mostly operating on things like constraints and heuristics, abstraction, and subproblems. (There may be some babbling and pruning in there, but it’s not doing most of the algorithmic efficiency work, and we're not directly babbling and pruning whole plans.)

In fact, even classic path search algorithms (like e.g. A* search) don’t really resemble pure babble and prune on closer inspection. When using a classic path-search algorithm to e.g. find a route from LA to to Seattle, we don’t actually babble lots of possible LA-Seattle routes and evaluate them. Rather, we come up with solutions to lots of *subproblems*: routes between LA and various possible intermediate points (like San Francisco or Sacramento or Portland). And in the case of A* search, we use heuristics (generated by constraint relaxation) to decide which subproblems to pay attention to.

So what is search, if not (as [Ivan Vendrov recently put it](https://www.lesswrong.com/posts/TdesHi8kkyokQdDoQ/gradient-descent-doesn-t-select-for-inner-search?commentId=Hi8CmzotoFYHiNoHW)) “enumerating possible actions and evaluating their consequences”? Well, I’d say that a “general-purpose search” process is something which:

*   Takes in a problem or goal specification (from a fairly broad range of possible problems/goals)
*   … and returns a plan which solves the problem or scores well on the goal

The “evaluation of consequences” thing is relevant, but it’s not really about the internal operations of the search process. Rather, “evaluation of consequences” is the defining feature of whether a search process is doing its job correctly: to tell whether the search process is working, use the problem/goal specification to evaluate the consequences of the plan generated, and see whether the plan solves the problem or scores well on the goal.

Note that “general-purpose search” still does not include all the pieces of general-purpose problem solving; there’s still things like gathering information, building and updating models, execution of plans, or recognizing when plans need to be updated. But it does include the planning part.

Also note that a general-purpose search process need not solve all possible problems, or even all compactly-specifiable problems; that would be NP-Hard (almost by definition). It does need to handle a broad range of problems, ideally in some realistic environment.

### Retargetability and Recursive Structure of Search

One key feature of a general-purpose search process is that it’s *retargetable*: because it can take in any problem or goal specification from a fairly wide class, we can retarget it by passing in a different problem/goal. For example, a path-finding algorithm (like A*) can take any start and end point in a graph.

Retargetability plays well with the recursive structure of search. When finding a route from LA to Seattle, for instance, we look for routes from LA to Sacramento or San Francisco. Those subproblems are themselves search problems, and can themselves be solved with the same path-finding method: just pass LA and Sacramento as the start and end points, rather than LA and Seattle. More generally, with a retargetable search process, we can recursively call the *same* general-purpose search process with different inputs in order to handle subproblems.

Later on, when we talk about why one might expect general-purpose search to eventually show up in trained ML systems, that sort of recursion will be one big piece.

Heuristics and Generality
-------------------------

In practice, in order to search *efficiently*, we tend to need some kind of heuristics. In physical-world pathfinding, for instance, a useful heuristic is to try to reduce Euclidean distance to the goal. However, that heuristic isn’t useful for all problems; Euclidean distance isn’t very useful as an heuristic for e.g. moderating online debates.

If we need heuristics for efficient search, and heuristics are specialized, how is general-purpose problem-solving a thing? Do we need to learn a whole new set of heuristics every time our task changes at all? Obviously not, in practice; most of the things on Ben’s list were things he only did once, but he nonetheless did a decent job (I know this because I’m one of his users). But then how do we achieve generality while relying on heuristics? Two main paths:

*   There exist general-purpose methods for generating heuristics
*   Heuristics tend to depend on the environment, but not on the exact objective

### General-Purpose Generators Of Heuristics Are A Thing

Probably the best-understood heuristic-generator is problem relaxation.

Here’s how it works. Let’s say I’m planning a trip to the grocery store. My planning problem has a whole bunch of constraints: I need to have enough time, I need to be at the right place, can’t let the frozen goods melt, the car must have enough gas, etc, etc. As a first pass, I ignore (a.k.. “relax”) most of those constraints, and only pay attention to a few - like e.g. having enough time. That narrows down my solution space a lot: there’s only a few spaces in my calendar with enough time. I pick a time slot. Then, I start to add other constraints back in.

The “relaxed” problem, in which I only worry about the time constraint, acts as an heuristic for the full problem. It helps me narrow down the search space early on, and focus on plans which at least satisfy that one constraint. And because it only involves one constraint, it’s relatively easy to solve the relaxed problem.

More generally, the steps of problem relaxation are roughly:

*   Start with a problem with a bunch of constraints
*   Relax (i.e. ignore) a bunch of the constraints
*   Solve the relaxed problem
*   Use the relaxed problem-solution as an heuristic for the full problem

Another example, this time from path search: if we’re solving a maze, we can relax the problem by ignoring all the walls. Then the shortest path is easy: it’s a straight line from the start to the end, and its length is Euclidean distance. We can then use Euclidean distance as an heuristic while exploring the maze: preferentially explore in directions which have shorter Euclidean distance to the end. In other words, preferentially explore directions for which the relaxed problem (i.e. ignoring all the walls) has the shortest solutions.

Problem relaxation probably isn’t the only heuristic generation method, but it’s relatively well-understood mathematically, and it’s an existence proof. It shows that general-purpose generators of heuristics are a thing. We should expect to see such heuristic-generators used inside of general-purpose search processes, in order to achieve generality and efficiency simultaneously.

### Heuristics Tend To Depend On The Environment, But Not On The Objective

The previous section talked about two heuristics:

*   When planning a grocery trip, pick a time slot while ignoring everything else
*   When path-finding, prioritize shorter Euclidean distance to the end

Notice that both of these heuristics are very environment-dependent, but not very goal-dependent. If my time is very scarce, then picking a time slot first will be a good heuristic for planning all sorts of things in my day-to-day life, from meetings to vacations to workouts to a trip to the movies to some quiet reading. The heuristic does depend on “the environment” - i.e. it would be less useful for someone whose time is abundant - but it’s relatively goal-agnostic.

Similarly, I can use Euclidean distance as a heuristic for finding road-trip routes between a wide variety of start and end locations. There are environments in which it’s a terrible heuristic, but for road-trip planning it works decently for most start/end points.

Here’s a visual example which I love, an heuristic for path-finding in a maze from an [old post](https://www.lesswrong.com/posts/CPBmbgYZpsGqkiz2R/problem-solving-with-mazes-and-crayon):

![](https://lh5.googleusercontent.com/AGSshp-ujjLfokcp8Toz0HilSFFRVHBicDBncjYqO49OIqEdQvAmgsNKWPhn-Aqzehhx1X9u0A9itvmig7qdGuFRSt4GLdb6oRAGBsjcvVUMM4XACVsZsSXnWAGjuYX1Bb5ujDOBf2QtogAesziB03g)

Maze-specific, but it’s useful for a wide variety of start/end points.

The pattern: heuristics tend to be environment-dependent but relatively goal-agnostic.

Why would such a pattern apply?

One way to frame the answer: an environment typically includes a bunch of stable constraints, shared across problems in that environment. In our universe, the laws of physics are all constraints, human laws are all constraints, [I can only move around so fast and carry so much](https://www.lesswrong.com/s/xEFeCwk3pdYdeG2rL/p/4s2gbwMHSdh2SByyZ), I only have so much time and money, my [interfaces](https://www.lesswrong.com/posts/8oMF8Lv5jiGaQSFvo/boundaries-part-1-a-key-missing-concept-from-utility-theory) with other people/things [only have certain knobs to turn](https://www.lesswrong.com/s/xEFeCwk3pdYdeG2rL/p/hyShz2ABiKX56j5tJ), etc. And [instrumental convergence](https://www.lesswrong.com/tag/instrumental-convergence) means that it will very often be the same few constraints which are rate-limiting. So, insofar as we generate heuristics via constraint relaxation, we’ll get environment-specific but reasonably goal-agnostic heuristics.

Another way to frame the answer: [natural abstraction](https://www.lesswrong.com/posts/cy3BhHrGinZCp3LXE/testing-the-natural-abstraction-hypothesis-project-intro). Most far-apart chunks of the world only interact via relatively low-dimensional summaries; the rest of their interaction is wiped out by noise. So, optimizing those low-dimensional summaries will be a common heuristic across a wide range of goals within the same environment.

### Side Note: Cached Solutions Are Not Heuristics (But Are Another General-Purpose Search Trick)

Another general-purpose search trick which someone will probably bring up if I don’t mention it is caching solutions to common subproblems. I don’t think of this as an heuristic; it mostly doesn’t *steer* the search process, just speed it up. But it is a useful and common trick for improving efficiency. And we should expect it to speed up search over a wide variety of goals within the same environment, insofar as [instrumental convergence](https://www.lesswrong.com/tag/instrumental-convergence) applies in that environment. Instrumentally convergent subproblems come up over and over again for a wide variety of problems, so caching solutions to those subproblems is a general-purpose search accelerator.

Revisiting The Risks From Learned Optimization Arguments
--------------------------------------------------------

We’ve now talked about how general-purpose search Is A Thing. We’ve talked about how general-purpose heuristics (and other general-purpose search tricks, like caching) Are A Thing. And we’ve observed that these things show up in humans. But why do they show up in humans? And should we expect them to show up in ML, or in intelligent aliens, or in other evolved/trained/selected agenty systems?

### Key Idea: Compression Is Favored By Default

In general, evolved/trained/selected systems favor more compact policies/models/heuristics/algorithms/etc. In ML, for instance, the fewer parameters needed to implement the policy, the more parameters are free to vary, and therefore the more parameter-space-volume the policy takes up and the more likely it is to be found. (This is also the main argument for why overparameterized ML systems are able to generalize at all.)

The outer training loop doesn't just select for high reward, it also implicitly selects for compactness. We expect it to find, not just policies which achieve high reward, but policies which are very compactly represented.

At the same time, assuming the system encounters a wide variety of problems in its training environment, it needs generality in order to perform well. But compactness means that, when possible, it will favor generality using a smaller number of more general pieces rather than a larger number of more specialized pieces.

So things like general-purpose heuristics, general-purpose heuristic generators, and general-purpose search are exactly the sort of things these systems should favor (assuming the architecture is expressive enough, and the environment varies enough).

That’s basically the argument for inner agents from [Risks From Learned Optimization](https://www.lesswrong.com/posts/q2rCMHNXazALgQpGH/conditions-for-mesa-optimization):

> In some tasks, good performance requires a very complex policy. At the same time, base optimizers are generally biased in favor of selecting learned algorithms with lower complexity. Thus, all else being equal, the base optimizer will generally be incentivized to look for a highly compressed policy.
> 
> One way to find a compressed policy is to search for one that is able to use general features of the task structure to produce good behavior, rather than simply memorizing the correct output for each input. A mesa-optimizer is an example of such a policy. From the perspective of the base optimizer, a mesa-optimizer is a highly-compressed version of whatever policy it ends up implementing: instead of explicitly encoding the details of that policy in the learned algorithm, the base optimizer simply needs to encode how to search for such a policy. Furthermore, if a mesa-optimizer can determine the important features of its environment at runtime, it does not need to be given as much prior information as to what those important features are, and can thus be much simpler.

On top of all that, the recursive nature of search - the fact that recursively searching on subproblems is useful as a search technique - favors a retargetable general-purpose search process, as opposed to a hardcoded optimizer.

Takeaways
---------

It seems like a lot of people have a picture of “search” as babble and prune. They correctly notice how inefficient babble and prune usually is, and [conclude that](https://www.lesswrong.com/posts/TdesHi8kkyokQdDoQ/gradient-descent-doesn-t-select-for-inner-search) it probably won’t be selected for in trained ML systems.

But there’s a more general notion of general-purpose search. It’s a process which takes in any problem or goal from a wide class, and returns a plan which solves the problem or achieves the goal. That’s the sort of search we expect to see show up in trained systems. The key property is that it’s *retargetable*: the search process can take in a wide variety of goals. That retargetability is selected for due to the recursive structure of search: recursively running search on subproblems is a widely-useful technique, and that technique can be encoded much more compactly with a retargetable search process on hand.

In order for that search to run efficiently, while still maintaining generality and compactness, it will probably need to either generate heuristics in a general way, or leverage goal-agnostic (but possibly environment-specific) heuristics. Both of those are plausible options: problem relaxation is an existence proof of general-purpose generators of heuristics, and the shared constraints of an environment (plus natural abstraction and/or instrumental convergence) offer a source of goal-agnostic heuristics.

Some strategic upshots of all this:

*   Possibility of rapid general capability gain if/when a system [groks](https://www.lesswrong.com/posts/N6WM6hs7RQMKDhYjB/a-mechanistic-interpretability-analysis-of-grokking) general-purpose search, especially if many general-purpose heuristics are learned first
*   [Retargeting the search process](https://www.lesswrong.com/posts/w4aeAFzSAguvqA5qu/how-to-go-from-interpretability-to-alignment-just-retarget) as an (inner) alignment strategy
*   Relatively high chance of [agent-like internal structure](https://www.lesswrong.com/posts/osxNg6yBCJ4ur9hpi/does-agent-like-behavior-imply-agent-like-architecture), like e.g. a reusable general-purpose search module which takes in an explicit objective