NLP as a study of representations

Ellen Riloff and I run an NLP reading group pretty much every semester. Last semester we covered "old school NLP." We independently came up with lists of what we consider some of the most important ideas (idea = paper) from pre-1990 (most are much earlier) and let students select which to present. There was a lot of overlap between Ellen's list and mine (not surprisingly). If people are interested, I can provide the whole list (just post a comment and I'll dig it up). The whole list of topics is posted as a comment. The topics that were actually selected are here.

I hope the students have found this exercise useful. It gets you thinking about language in a way that papers from the 2000s typically do not. It brings up a bunch of issues that we no longer think about frequently. Like language. (Joking.) (Sort of.)

One thing that's really stuck out for me is how much "old school" NLP comes across essentially as a study of representations. Perhaps this is a result of the fact that AI -- as a field -- was (and, to some degree, still is) enamored with knowledge representation problems. To be more concrete, let's look at a few examples. It's already been a while since I read these last (I had meant to write this post during the spring when things were fresh in my head), so please forgive me if I goof a few things up.

I'll start with one I know well: Mann and Thompson's rhetorical structure theory paper from 1988. This is basically "the" RST paper. I think that when a many people think of RST, they think of it as a list of ways that sentences can be organized into hierarchies. Eg., this sentence provides background for that one, and together they argue in favor of yet a third. But this isn't really where RST begins. It begins by trying to understand the communicative role of text structure. That is, when I write, I am trying to communicate something. Everything that I write (if I'm writing "well") is toward that end. For instance, in this post, I'm trying to communicate that old school NLP views representation as the heart of the issue. This current paragraph is supporting that claim by providing a concrete example, which I am using to try to convince you of my claim.

As a more detailed example, take the "Evidence" relation from RST. M+T have the following characterization of "Evidence." Herein, "N" is the nucleus of the relation, "S" is the satellite (think of these as sentences), "R" is the reader and "W" is the writer:

relation name: Evidence
constraints on N: R might not believe N to a degree satisfactory to W
constraints on S: R believes S or will find it credible
constraints on N+S: R's comprehending S increases R's belief of N
the effect: R's belief of N is increased
locus of effect: N

This is a totally different way from thinking about things than I think we see nowadays. I kind of liken it to how I tell students not to program. If you're implementing something moderately complex (say, forward/backward algorithm), first write down all the math, then start implementing. Don't start implementing first. I think nowadays (and sure, I'm guilty!) we see a lot of implementing without the math. Or rather, with plenty of math, but without a representational model of what it is that we're studying.

The central claim of the RST paper is that one can think of texts as being organized into elementary discourse units, and these are connected into a tree structure by relations like the one above. (Or at least this is my reading of it.) That is, they have laid out a representation of text and claimed that this is how texts get put together.

As a second example (this will be sorter), take Wendy Lehnert's 1982 paper, "Plot units and narrative summarization." Here, the story is about how stories get put together. The most interesting thing about the plot units model to me is that it breaks from how one might naturally think about stories. That is, I would naively think of a story as a series of events. The claim that Lehnert makes is that this is not the right way to think about it. Rather, we should think about stories as sequences of affect states. Effectively, an affect state is how a character is feeling at any time. (This isn't quite right, but it's close enough.) For example, Lehnert presents the following story:

When John tried to start his care this morning, it wouldn't turn over. He asked his neighbor Paul for help. Paul did something to the carburetor and got it going. John thanked Paul and drove to work.

The representation put forward for this story is something like: (1) negative-for-John (the car won't start), which leads to (2) motivation-for-John (to get it started, which leads to (3) positive-for-John (it's started), when then links back and resolves (1). You can also analyze the story from Paul's perspective, and then add links that go between the two characters showing how things interact. The rest of the paper describes how these relations work, and how they can be put together into more complex event sequences (such as "promised request bungled"). Again, a high level representation of how stories work from the perspective of the characters.

So now I, W, hope that you, R, have an increased belief in the title of the post.

Why do I think this is interesting? Because at this point, we know a lot about how to deal with structure in language. From a machine learning perspective, if you give me a structure and some data (and some features!), I will learn something. It can even be unsupervised if it makes you feel better. So in a sense, I think we're getting to a point where we can go back, look at some really hard problems, use the deep linguistic insights from two decades (or more) ago, and start taking a crack at things that are really deep. Of course, features are a big problem; as a very wise man once said to me: "Language is hard. The fact that statistical association mining at the word level made it appear easy for the past decade doesn't alter the basic truth. :-)." We've got many of the ingredients to start making progress, but it's not going to be easy!