[Edited, with a substantial addition, August 2, 2020]
The purpose of this post is to sketch a conceptual framework in which we can understand the success of language models such as GPT-3 despite the fact that they are based on nothing more than massive collections of bare naked signifiers. There’s not a signified in sight, much less any referents. I have no intention of even attempting to explain how GPT-3 works. That it does work, in an astonishing variety of cases if (certainly) not universally, is sufficient for my purposes.
First of all I present the insight that sent me down this path, a comment by Graham Neubig in an online conversation that I was not a part of. Then I set that insight in the context of and insight by Sydney Lamb (meaning resides in relations), a first-generation researcher in machine translation and computational linguistics. I think take a grounding case by Julian Michael, that of color, and suggest that it can be extended by the work of Peter Gärdenfors on conceptual spaces.
A clue: an isomorphic transform into meaning space
At the 58th Annual Meeting of the Association for Computational Linguistics Emily M. Bender and Alexander Koller delivered a paper, Climbing towards NLU: On Meaning, Form, and Understanding in the Age of Data [1], where NLU means natural language understanding. The issue is pretty much the one I laid out in my previous posts in the sections “No words, only signifiers” and “Martin Kay, ‘an ignorance model’” [2]. A lively discussion ensured online which Julian Michael has summarized and commented on in a recent blog post [3].
In that post Michael quotes a remark by Graham Neubig:
One thing from the twitter thread that it doesn’t seem made it into the paper... is the idea of how pre-training on form might learn something like an “isomorphic transform” onto meaning space. In other words, it will make it much easier to ground form to meaning with a minimal amount of grounding. There are also concrete ways to measure this, e.g. through work by Lena Voita or Dani Yogatama... This actually seems like an important point to me, and saying “training only on form cannot surface meaning,” while true, might be a little bit too harsh— something like “training on form makes it easier to surface meaning, but at least a little bit of grounding is necessary to do so” may be a bit more fair.
That’s my point of departure in this post, that notion of “an ‘isomorphic transform’ onto meaning space.” I am going to sketch a framework in which we can begin unpacking that idea. But it may take awhile to get there.
Meaning is in relations
I want to develop an idea I have from Sydney Lamb, that meaning resides in relations. The idea is grounded in the “old school” world of symbolic computation, where language is conceived as a relational network of items. The meaning of any item in the network is a function of its position in the network.
Let’s start with this simple diagram:
It represents the fact that the central nervous system (CNS) is coupled to two worlds, each external to it. To the left we have the external world. The CNS is aware of that world through various senses (vision, hearing, smell, touch, taste, and perhaps others) and we act in that world through the motor system. But the CNS is also coupled to the internal milieu, with which it shares a physical body. The net is aware of that milieu by chemical sensors indicating contents of the blood stream and of the lungs, and by sensors in the joints and muscles. And it acts in the world through control of the endocrine system and the smooth muscles. Roughly speaking the CNS guides the organism’s actions in the external world so as to preserve the integrity of the internal milieu. When that integrity is gone, the organism is dead.
Now consider this more differentiated presentation of the same facts:
I have divided the CNS into four sections: A) senses the external world, B) senses the internal milieu, D) guides action in the internal milieu, and D) guides action in the external world. I rather doubt that even a very simple animal, such as C. elegans, with 302 neurons, is so simple. But I trust my point will survive that oversimplification.
Lamb’s point is that the “meaning” or “significance” of any of those nodes – let’s not worry at the moment whether they’re physical neurons or more abstract entities – is a function of its position in the entire network, with its inputs from and outputs to the external world and the inner milieu [4]. To appreciate the full force of Lamb’s point we need to recall the diagrams typical of old school symbolic computing, such as this diagram from Brian Phillips we used in the previous post:

All of the nodes and edges have labels. Lamb’s point is that those labels exist for our convenience, they aren’t actually a part of the system itself. If we think of that network as a fragment from a human cognitive system – and I’m pretty sure that’s how Phillips thought about it, even if he could not justify it in detail (no one could, not then, not now) – then it is ultimately connected to both the external world and the inner milieu. All those labels fall away; they serve no purpose. Alas, Phillips was not building a sophisticated robot, and so those labels are necessary fictions.
But we’re interested in the full real case, a human being making their way in the world. In that case let us assume that, for one thing, the necessary diagram is WAY more complex, and that the nodes and edges do not represent individual neurons. Rather, they represent various entities that are implemented in neurons, sensations, thoughts, perceptions, and so forth. Just how such things are realized in neural structures is, of course, a matter of some importance and is being pursued by hundreds of thousands of investigators around the world. But we need not worry about that now. We’re about to fry some rather more abstract fish (if you will).
Some of those nodes will represent signifiers, to use the Saussurian terminology I used in my previous post, and some will represent signifieds. What’s the difference between a signifier and a signified? Their position in the network as a whole. That’s all. No more, no less. Now, it seems to me, we can begin thinking about Neubig’s “isomorphic transform” onto meaning space.
Let us notice, first of all, that language exists as strings of signifiers in the external world. In the case that interests us, those are strings of written characters that have been encoded into computer-readable form. Let us assume that the signifieds – which bear a major portion of meaning, no? – exist in some high dimensional network in mental space. This is, of course, an abstract space rather than the physical space of neurons, which is necessarily three dimensional. However many dimensions this mental space has, each signified exists at some point in that space and, as such, we can specify that point by a vector containing its value along each dimension.
What happens when one writes? Well, one produces a string of signifiers. The distance between signifiers on this string, and their ordering relative to one another, are a function of the relative distances and orientations of their associated signifieds in mental space. That’s where to look for Neubig’s isometric transform into meaning space. What GPT-3, and other NLP engines, does is to examine the distances and ordering of signifiers in the string and compute over them so as to reverse engineer the distances and orientations of the associated signifieds in high-dimensional mental space.
[A little reflection on that formulation makes it clear that it fails to take into account a distinction central to ‘old school’ symbolic computation, that between semantic and episodic memory. Rather than interrupt this argument with a refined formulation I have placed that in an appendix to this post: A more refined approach to meaning space. I also offer some remarks on need for a connection to the physical world in order to handle common-sense reasoning.]
Is the result perfect? Of course not – but then how do we really know? It’s not as though we’ve got a well-accepted model of human conceptual space just lying around on a shelf somewhere. GPT-3’s language model is perhaps as good as we’ve got at the moment, and we can’t even open the hood and examine it. We know its effectiveness by examining how it performs. And it performs very well.