Saturday, December 21, 2013

You Think Too Much

This first appeared as one of my few facebook notes. I'm copying it here; perhaps it's a better place for it.

This is what the phrase "You're overthinking it" is like.

Mary and Bob sit down to a game of chess. Mary is an inexperienced player, and Bob is giving her some advice. He's being nice, trying to hint at the right moves without telling her what to do. So, Bob is making his moves very quickly, since he's experienced, and Mary is not difficult to play against (and he's going easy on her anyway). Mary is very uncertain of most of her moves, and spends a lot of time staring at the board indecisively.

At one point, Mary is looking at the board in confusion. Bob sees very plainly what move she needs to make; she should move her rook to defend her knight. Mary is looking very carefully at all the possible moves she could make, trying hard to evaluate which things might be good or bad for her, trying to think a few moves ahead.

"You're thinking too much," Bob says. "It's very simple."

This advice sounds helpful to Bob. From Bob's perspective, Mary is spending a lot of time thinking about many alternatives when she should be quickly hitting on the critical move. And it's true: if Mary were a better player, she would be thinking less here.

From Mary's perspective, this is not very helpful at all. She tries to take Bob's advice. She tries to think less about her move. She figures, if Bob says "It's simple", this must mean that she doesn't need to look several moves ahead to see the consequences. She looks for possible moves again, this time looking for things that have good consequences for her immediately.

Mary moves the pawn up to threaten one of Bob's pieces.

Bob takes Mary's knight.

Bob explains to a frustrated Mary what she could have done to avoid this. "See? You're overthinking it" he adds. To Bob, this feels like the right explanation for Mary's wrong move: she was thinking about all these other pieces, when she needed to be defending her knight.

The worst part is, Mary starts to be convinced, too. She admits that she was taking a lot of time to look several moves ahead in all kinds of situations that turned out to be irrelevant to what she needed to do. She tries to think less during the rest of the game, and makes many other mistakes as a result.

Sunday, December 15, 2013

Where is AI Headed?

I've spent a lot of effort on this blog arguing for the direction of higher expressiveness. Machine intelligence should be able to learn anything a human can learn, and in order for that to be possible, it should be able to conceive of any concept that a human can. I have proceeded with the belief that this is the direction to push in order for the field to make progress.

Yet, in some ways at least, the field is headed in the opposite direction.

I've often discussed the Chomsky hierarchy, and how most techniques at present fall very low on it. I've often discussed hierarchies "above" the Chomsky hierarchy; hierarchies of logic & truth, problems of uncomputability and undefinability. Reaching for the highest expression of form, the most general notion of pattern.

Machine learning has made artificial intelligence increasingly practical. Yet, the most practical techniques are often the least expressively powerful. Machine learning flourished once it abandoned the symbolic obsession of GOFAI. Fernando Pereira famously said: "The older I get, the further down the Chomsky Hierarchy I go."

There's a good reason for this, too. Highly structured techniques like logic induction and genetic programming (both of which would go high in the hierarchy) don't scale well. Commercial machine learning is large-scale, and increasingly so. I mentioned this in connection with word2vec last time: "Using very shallow learning makes the technique faster, allowing it to be trained on (much!) larger amounts of data. This gives a higher-quality result." 

The "structure" I'm referring to provides more prior bias, which means more generalization capability. This is very useful when we want to come to the correct conclusion using small amounts of data. However, with more data, we can cover more and more cases without needing to actually make the generalization. At some point, the generalization becomes irrelevant in practice.

Take XML data. You can't parse XML with regular expressions.1 Regular expressions are too low on the Chomsky hierarchy to form a proper model of what's going on. However, for the Large Text Compression Benchmark, which requires us to compress XML data, the leading technique is the PAQ compressor. Compression is equivalent to prediction, so the task amounts to making a predictive model of XML data. PAQ works by constructing a probabilistic model of the sequence of bits, similar to a PPM model. This is not even capable of representing regular expressions. Learning regular expressions is like learning hidden markov models. PPM allows us to learn fully observable markov models. PAQ learns huge markov models that get the job done.

The structure of XML requires a recursive generalization, to understand the nested expressions. Yet, PAQ does acceptably well, because the depth of the recursion is usually quite low.

You can always push a problem lower down on the hierarchy if you're willing to provide more data (often exponentially more), and accept that it will learn the common cases and can't generalize the patterns to the uncommon ones. In practice, it's been an acceptable loss.

Part of the reason for this is that the data just keeps flowing. The simpler techniques require exponentially more data... and that's how much we're producing. It's only getting worse:

Has Big Data Made Anonymity Impossible? MIT Technology Review
At The New Yorker, Gary Marcus complains: Why Can't My Computer Understand Me? Reviewing the work of Hector Levesque, the article conveys a desire to "google-proof" AI, designing intelligence tests which are immune to the big-data approach. Using big data rather than common-sense logic to answer facts is seen as cheating. Levesque presents a series of problems which cannot (presently) be solved by such techniques, and calls others to "stop bluffing".

I can't help but agree. Yet, it seems the tide of history is against us. As the amount of data continues to increase, dumb techniques will achieve better and better results.

Will this trend turn around at some point?

Gary Marcus points out that some information just isn't available on the web. Yet, this is a diminishing reality. As more and more of our lives are online (and as the population rises), more and more will be available in the global brain.

Artificial intelligence is evolving into a specific role in that global brain: a role which requires only simple association-like intelligence, fueled by huge amounts of data. Humans provide the logically structured thoughts, the prior bias, the recursive generalizations; that's a niche which machines are not currently required to fill. At the present, this trend only seems to be increasing.

Should we give up structured AI?

I don't think so. We can forge a niche. We can climb the hierarchy. But it's not where the money is right now... and it may not be for some time.

1: Cthulhu will eat your face.

Monday, December 9, 2013

History of Distributed Representations

Commenting on the previous post, a friend pointed out that "distributed representations" are not so new. I thought I would take a look at the history to clarify the situation.

In a very broad sense, I was discussing the technique of putting a potentially nonlinear problem into a linear vector space. This vague idea matches to many techniques in machine learning. A number of well-developed algorithms take advantage of linearity assumptions, including PCA, logistic regression, and SVM.1 A common approach to machine learning is to find a number of features, which are just functions of your data, and use one of these techniques on the features (hoping they are close enough to linear). Another common technique, the kernel trick, projects features into a higher-dimensional space where the linearity assumption is more likely to get good results. Either way, a large part of the work to get good results is "feature engineering": choosing how to represent the data as a set of features to feed into the machine learning algorithm.

We could even argue that probability theory itself is an example: probabilities are always linear, no matter how nonlinear the underlying problem being described. (The probability of an event is the sum of the ways it could happen.) This gives us nice results; for example, there is always a Nash equilibrium for a game if we allow probabilistic strategies. This is not the case if we only consider "pure" strategies.

This theme is interesting to me, but, I was trying to be much more narrow in talking about recent developments in distributed representations. Like feature-based machine learning, a distributed representation will put data into a vector space to make it easier to work with. Unlike approaches relying on feature engineering, there is an emphasis on figuring out how to get the representations to "build themselves", often starting with randomly assigned vector representations.

The beginning of this kind of approach is probably latent semantic analysis (LSA), which is from 1988. LSA assigns 'semantic vectors' to words based on statistical analysis of the contexts those words occur in, based on the idea that words with similar meaning will have very similar statistics.

Given how old this technique is, the excitement around Google's release of the word2vec tool is striking. Reports spun it as deep learning for the masses. Deep learning is a much more recent wave of development. I think the term has lost much of its meaning in becoming a buzzword.2 Calling word2vec "deep" takes this to farcical levels: the techniques of word2vec improve previous models by removing the hidden layer from the network. Using very shallow learning makes the technique faster, allowing it to be trained on (much!) larger amounts of data. This gives a higher-quality result.

One of the exciting things about word2vec is the good results with solving word analogies by vector math. The result of vector computations like France - Paris and Russia - Moscow are very similar, meaning we can approximately find the vector for a capital given the vector for the corresponding nation. The same trick works for a range of word relationships.

However, I've talked with people who had the incorrect impression that this is a new idea. I'm not sure exactly how old it is, but I've heard the idea mentioned before, and I did find a reference from 2004 which appears to use LSA to do the same basic thing. (I can't see the whole article on google books...)

One thing which I thought was really new was the emerging theme of combining vectors to form representations of compound entities. This, too, is quite old. I found a paper from 1994, which cites harder-to-find papers from 1993, 1990, and 1989 that also developed techniques to combine vectors to create representations of compound objects. Recent developments seem much more useful, but, the basic idea is present.

So, all told, it's a fairly long-standing area which has seen large improvements in the actual techniques employed, but, whose central ideas were laid out (in one form or another) over 20 years ago.

1: By the way, don't get too hung up about what makes one machine learning technique "linear" and another "nonlinear". This is a false dichotomy. What I really mean is that a technique works in a vector space (which more or less means a space where + is defined and behaves very much like we expect), and relies "largely" on linear operations in this space. What does "linear" actually mean? A function F is linear if and only if F(x+y) = F(x) + F(y) and for scalar a, F(ax) = aF(x). PCA, for example, is justified by minimizing a squared error (a common theme), where the error is based on euclidean distance, a linear operation. Notice that taking the square isn't linear, but PCA is still thought of as a linear approach.

2: Deep learning has come to mean almost any multi-layer neural network. The term caught on with the success related to Deep Belief Networks, which proposed specific new techniques. Things currently being called "deep learning" often have little in common with this. I feel the term has been watered down by people looking to associate their work with the success of others. This isn't all bad. The work on multi-layered networks seems to have produced real progress in reducing or eliminating the need for feature engineering.

Tuesday, December 3, 2013

Distributed Representations

Distributed vector representations are a set of techniques which take a domain (usually, words) and embed it into a linear space (representing each word as a large vector of numbers). Useful tasks can then be represented as manipulations of these embedded representations. The embedding can be created in a variety of ways; often, it is learned by optimizing task performance. SENNA demonstrated that representations learned for one task are often useful for others.

There are so many interesting advances being made in distributed vector representations, it seems that a nice toolset is emerging which will soon be considered a basic part of machine intelligence.

Google's word2vec assigns distributed vector representations to individual words and a few short phrases. These representations have been shown to give intuitively reasonable results on analogy tasks with simple vector math: king - man + woman is approximately equal to the vector for queen, for example. This is despite not being explicitly optimized for that task, again showing that these representations tend to be useful for a wide range of tasks.

Similar approaches have aided machine translation tasks by turning word translation into a linear transform from one vector space to another.

One limitation of this approach is that we cannot do much to represent sentences. Sequences of words can be given somewhat useful representations by adding together the individual word representations, but this approach is limited.

Socher's RNN learns a matrix transform to compose two elements together and give them a score, which is then used for greedy parsing by composing together the highest-scoring items, with great success. This gives us useful vector representations for phrases and sentences.

Another approach which has been suggested is circular convolution. This combines vectors in a way which captures ordering information, unlike addition or multiplication. Impressively, the technique has solved Raven progressive matrix problems:

Then there's a project, COMPOSES, which seeks to create a language representation in which nouns get vector representations and other parts of speech get matrix representations (and possibly tensor representations?).

I haven't looked into the details fully, but conceptually it makes sense: the parts of speech which intuitively represent modifiers are linear functions, while the parts of speech which are intuitively static objects are getting operated on by these functions.

The following paper gives a related approach:

Here, everything is represented as a matrix of the same size. Representing the objects as functions is somewhat limiting, but the uniform representation makes it easy to jump to higher-level functions (modifiers on modifiers) without adding anything. This seems to have the potential to enable a surprisingly wide range of reasoning capabilities, given the narrow representation.

As the authors of that last paper mention, the approach can only support reasoning of a "memorized" sort. There is no mechanism which would allow chained logical inferences to answer questions. This seems like a good characterization of the general limitations of the broader set of techniques. The distributed representation of a word, phrase, image, or other object is a static encoding which represents, in some sense, a classification of the object into a fuzzy categorization system we've learned. How can we push the boundary here, allowing for a more complex reasoning? Can these vector representations be integrated into a more generally capable probabilistic logic system?