The following is a conversation with John Hopfield, professor at Princeton, whose life's work weaved beautifully through biology, chemistry, neuroscience, and physics. Most crucially, he saw the messy world of biology through the piercing eyes of a physicist. He's perhaps best known for his work on associative neural networks, now known as Hopfield networks, that were one of the early ideas that catalyzed the development of the modern field of deep learning.

As his 2019 Franklin Medal in Physics Award states, he applied concepts of theoretical physics to provide new insights on important biological questions in a variety of areas, including genetics and neuroscience, with significant impact on machine learning. And as John says in his 2018 article titled, "Now What?" his accomplishments have often come about by asking that very question, now what?

And often responding by a major change of direction. This is the Artificial Intelligence Podcast. If you enjoy it, subscribe on YouTube, give it five stars on Apple Podcast, support it on Patreon, or simply connect with me on Twitter. And Lex Friedman, spelled F-R-I-D-M-A-N. As usual, I'll do one or two minutes of ads now, and never any ads in the middle that can break the flow of the conversation.

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What difference between biological neural networks and artificial neural networks is most captivating and profound to you? At the higher philosophical level, let's not get technical just yet. One of the things that very much intrigues me is the fact that neurons have all kinds of components, properties to them. And evolutionary biology, if you have some little quirk in how a molecule works or how a cell works, and it can be made use of, evolution will sharpen it up and make it into a useful feature rather than a glitch.

And so you expect, in neurobiology, for evolution to have captured all kinds of possibilities of getting neurons, of how you get neurons to do things for you. And that aspect has been completely suppressed in artificial neural networks. - Do the glitches become features in the biological neural network? - They can.

Look, let me take one of the things that I used to do research on. If you take things which oscillate, they have rhythms which are sort of close to each other, under some circumstances, these things will have a phase transition, and suddenly the rhythm will, everybody will fall into step.

There was a marvelous physical example of that in the Millennium Bridge across the Thames River built about 2001. And pedestrians walking across, pedestrians don't walk, synchronize, they don't walk in lockstep, but they're all walking about the same frequency. And the bridge could sway at that frequency, and the slight sway made pedestrians tend a little bit to lock into step, and after a while, the bridge was oscillating back and forth and the pedestrians were walking in step to it.

You could see it in the movies made out of the bridge. And the engineers made a simple, minor mistake. They assumed when you walk, it's step, step, step, and it's back and forth motion. But when you walk, it's also right foot, left foot, side to side motion. And it's the side to side motion for which the bridge was strong enough, but it wasn't stiff enough.

And as a result, you would feel the motion and you'd fall into step with it. And people were very uncomfortable with it. They closed the bridge for two years while they built stiffening for it. Now, nerves, look, nerve cells produce action potentials. You have a bunch of cells which are loosely coupled together producing action potentials of the same rate.

There'll be some circumstances under which these things can lock together. Other circumstances in which they won't. Well, they fire together, you can be sure that other cells are gonna notice it. So you can make a computational feature out of this in an evolving brain. Most artificial neural networks don't even have action potentials, let alone have the possibility for synchronizing them.

- And you mentioned the evolutionary process. So the evolutionary process that builds on top of biological systems leverages that the weird mess of it somehow. So how do you make sense of that ability to leverage all the different kinds of complexities in the biological brain? - Well, look, at the biological molecule level, you have a piece of DNA which encodes for a particular protein.

You could duplicate that piece of DNA and now one part of it can code for that protein, but the other one could itself change a little bit and thus start coding for a molecule which is slightly different. Now that molecule was just slightly different, had a function which helped any old chemical reaction was as important to the cell.

You would go ahead and let that try and evolution would slowly improve that function. And so you have the possibility of duplicating and then having things drift apart. One of them retain the old function, the other one do something new for you. And there's evolutionary pressure to improve. Look, there is in computers too, but it's improvement has to do with closing some companies and opening some others.

(Lex laughing) The evolutionary process looks a little different. - Yeah, similar timescale perhaps. - Much shorter in timescale. - Companies close, yeah, go bankrupt and are born. - Yeah, shorter, but not much shorter. Some company lasts a century, but yeah, you're right. I mean, if you think of companies as a single organism that builds and you all know, yeah, it's a fascinating dual correspondence there between biological- - And companies have difficulty having a new product competing with an old product.

- Yeah. And when IBM built its first PC, you probably read the book. They made a little isolated internal unit to make the PC. And for the first time in IBM's history, they didn't insist that you build it out of IBM components, but they understood that they could get into this market, which is a very different thing by completely changing their culture.

And biology finds other markets in a more adaptive way. - Yeah, it's better at it. It's better at that kind of integration. So maybe you've already said it, but what to use the most beautiful aspect or mechanism of the human mind? Is it the adaptive, the ability to adapt as you've described or is there some other little quirk that you particularly like?

- Adaptation is everything when you get down to it. But the difference, there are differences between adaptation where you're learning goes on only over generations and over evolutionary time, where you're learning goes on at the time scale of one individual who must learn from the environment during that individual's lifetime.

And biology has both kinds of learning in it. And the thing which makes neurobiology hard is that it's a mathematical system, as it were, built on this other kind of evolutionary system. - What do you mean by mathematical system? Where's the math and the biology? - Well, when you talk to a computer scientist about neural networks, it's all math.

The fact that biology actually came about from evolution and the fact that biology is about a system which you can build in three dimensions. If you look at computer chips, computer chips are basically two-dimensional structures, maybe 2.1 dimensions, but they really have difficulty doing three-dimensional wiring. Biology is, the neocortex is actually also sheet-like, and it sits on top of the white matter, which is about 10 times the volume of the gray matter and contains all what you might call the wires.

But there's a huge, the effect of computer structure on what is easy and what is hard is immense. - So-- - And biology does, it makes some things easy that are very difficult to understand how to do computationally. On the other hand, you can't do simple floating-point arithmetic, so it's awfully stupid.

- Yeah, and you're saying this kind of three-dimensional complicated structure makes, it's still math, it's still doing math. The kind of math it's doing enables you to solve problems of a very different kind. - That's right, that's right. - So you mentioned two kinds of adaptation, the evolutionary adaptation and the adaptation, or learning at the scale of a single human life.

Which do you, which is particularly beautiful to you and interesting from a research and from just a human perspective? And which is more powerful? - I find things most interesting that I begin to see how to get into the edges of them and tease them apart a little bit and see how they work.

And since I can't see the evolutionary process going on, I'm in awe of it, but I find it just a black hole as far as trying to understand what to do. And so in a certain sense, I'm in awe of it, but I couldn't be interested in working on it.

- The human life's time scale is however thing you can tease apart and study. - Yeah, you can do, there's the developmental neurobiology which understands how the connections and how the structure evolves from a combination of what the genetics is like and the real, the fact that you're building a system in three dimensions.

- In just days and months, those early, early days of a human life are really interesting. - They are, and of course, there are times of immense cell multiplication. There are also times of the greatest cell death in the brain is during infancy. - It's turnover. - So what is not effective, what is not wired well enough to use the moment, throw it out.

- It's a mysterious process. - From, let me ask, from what field do you think the biggest breakthroughs in understanding the mind will come in the next decades? Is it neuroscience, computer science, neurobiology, psychology, physics, maybe math, maybe literature? (laughs) - Well, of course, I see the world always through a lens of physics.

I grew up in physics. And the way I pick problems is very characteristic of physics and of an intellectual background, which is not psychology, which is not chemistry and so on and so on. - Now, both of your parents are physicists. - Both of my parents were physicists, and the real thing I got out of that was a feeling that the world is an understandable place.

And if you do enough experiments and think about what they mean and structure things, or you can do the mathematics of the relevant to the experiments, you also be able to understand how things work. - But that was a few years ago. Did you change your mind at all through many decades of trying to understand the mind, of studying it different kinds of ways, not even the mind, just biological systems?

You still have hope that physics, that you can understand? - There's a question of what do you mean by understand? - Of course. - When I taught freshman physics, I used to say, I wanted to give physics to understand the subject, to understand Newton's laws. I didn't want them simply to memorize a set of examples to which they knew the equations to write down to generate the answers.

I had this nebulous idea of understanding. So that if you looked at a situation, you could say, oh, I expect the ball to make that trajectory, or I expect some intuitive notion of understanding. And I don't know how to express that very well. I've never known how to express it well.

And you run smack up against it, when you do these, look at these simple neural nets, feed forward neural nets, which do amazing things, and yet you know, contain nothing of the essence of what I would have felt was understanding. Understanding is more than just an enormous lookup table.

- Let's linger on that. How sure you are of that? What if the table gets really big? So, I mean, ask another way, these feed forward neural networks, do you think they'll ever understand? - Could answer that in two ways. I think if you look at real systems, feedback is an essential aspect of how these real systems compute.

On the other hand, if I have a mathematical system with feedback, I know I can unlayer this and do it, but I have an exponential expansion in the amount of stuff I have to build if I can solve the problem that way. - So feedback is essential. So we can talk even about recurrent neural networks, so recurrence, but do you think all the pieces are there to achieve understanding through these simple mechanisms?

Like, back to our original question, what is the fundamental, is there a fundamental difference between artificial neural networks and biological, or is it just a bunch of surface stuff? - Suppose you ask a neurosurgeon, when is somebody dead? - Yeah. - They'll probably go back to saying, well, I can look at the brain rhythms and tell you this is a brain which is never gonna function again.

This one is, this other one is one which if we treat it well, is still recoverable. And then just do that by some electrodes and looking at simple electrical patterns which don't look in any detail at all at what individual neurons are doing. These rhythms are utterly absent from anything which goes on at Google.

- Yeah, but the rhythms. - But the rhythms what? - So, well, that's like comparing, okay, I'll tell you. It's like you're comparing the greatest classical musician in the world to a child first learning to play. The question I'm at, but they're still both playing the piano. I'm asking, is there, will it ever go on at Google?

Do you have a hope? Because you're one of the seminal figures in both launching both disciplines, both sides of the river. - I think it's going to go on generation after generation the way it has where what you might call the AI computer science community says, let's take the following.

This is our model of neurobiology at the moment. Let's pretend it's good enough and do everything we can with it. And it does interesting things. And after the while it sort of grinds into the sand and you say, ah, something else is needed for neurobiology and some other grand thing comes in and enables you to go a lot further.

But we'll go into the sand again. And I think it's going to be generations of this evolution. I don't know how many of them and each one is going to get you further into what our brain does. And in some sense, pass the Turing test longer and more broad aspects.

And how many of these are good there are going to have to be before you say, I've made something, I've made a human, I don't know. - But your sense is it might be a couple. - My sense is it might be a couple more. - Yeah. - And going back to my brain waves as it were.

- Yes. - From the AI point of view, they would say, ah, maybe these are an heavy phenomenon and not important at all. The first car I had, a real wreck of a 1936 Dodge go above 45 miles an hour and the wheels was shimmy. - Yeah. - Good speedometer that.

Now, nobody designed the car that way. The car is malfunctioning to have that. But in biology, if it were useful to know when are you going more than 45 miles an hour, you just capture that and you wouldn't worry about where it came from. - Yeah. - It's going to be a long time before that kind of thing, which can take place in large complex networks of things is actually used in the computation.

Look, how many transistors are there in your laptop these days? - Actually, I don't know the number. It's-- - It's on the scale of 10 to the 10. I can't remember the number either. - Yeah. - And all the transistors are somewhat similar. And most physical systems with that many parts, all of which are similar, have collective properties.

- Yes. - Sound waves in air, earthquakes, what have you have collective properties, weather. There are no collective properties used in artificial neural networks in AI. - Yeah, it's very-- - If biology uses them, it's going to take us to more generations of things for people to actually dig in and see how they are used and what they mean.

- See, you're very right. We might have to return several times to neurobiology and try to make our transistors more messy. - Yeah, yeah. At the same time, the simple ones will conquer big aspects. And I think one of the most biggest surprises to me was how well learning systems, which are manifestly non-biological, how important they can be actually, and how important and how useful they can be in AI.

- So if we can just take a stroll to some of your work, that is incredibly surprising that it works as well as it does, that launched a lot of the recent work with neural networks. If we go to what are now called Hopfield networks, can you tell me what is associative memory in the mind for the human side?

Let's explore memory for a bit. - Okay, what you mean by associative memory is how you have a memory of each of your friends. Your friend has all kinds of properties from what they look like, to what their voice sounds like, to where they went to college, where you met them, go on and on, what science papers they've written.

If I start talking about a five foot 10 wire-aided cognitive scientist that's got a very bad back, it doesn't take very long for you to say, oh, he's talking about Jeff Hinton. I never mentioned the name or anything very particular, but somehow a few facts that are associated with a particular person enables you to get a hold of the rest of the facts, or not the rest of them, another subset of them.

And it's this ability to link things together, link experiences together, which goes under the general name of associative memory. And a large part of intelligent behavior is actually just large associative memories at work, as far as I can see. - What do you think is the mechanism of how it works in the mind?

Is it a mystery to you still? Do you have inklings of how this essential thing for cognition works? - What I made 35 years ago was of course a crude physics model to show the kind, actually enable you to understand, my old sense of understanding as a physicist, because you could say, ah, I understand why this goes to stable states.

It's like things going downhill. - Right. - And that gives you something with which to think in physical terms rather than only in mathematical terms. - So you've created these associative artificial networks. - That's right. And now if you look at what I did, I didn't at all describe a system which gracefully learns.

I described a system in which you could understand how learning could link things together, how very crudely it might learn. One of the things which intrigues me as I reinvestigate that system now to some extent is, look, I'll see you every second for the next hour or what have you.

Each look at you is a little bit different. I don't store all those second by second images. I don't store 3000 images. I somehow compact this information. So I now have a view of you, which I can use. It doesn't slavishly remember anything in particular, but it compacts the information into useful chunks, which are somehow, it's these chunks, which are not just activities of neurons, bigger things than that, which are the real entities which are useful to you.

- Useful to you to describe, to compress this information coming at you. - And you have to compress it in such a way that if the information comes in just like this again, I don't bother to rewrite it, or efforts to rewrite it simply do not yield anything because those things are already written.

And that needs to be not, look this up, have I written this, have I stored it somewhere already? It's gotta be something which is much more automatic in the machine hardware. - Right, so in the human mind, how complicated is that process, do you think? So you've created, feels weird to be sitting with John Hopfield calling him Hopfield Networks, but-- - It is weird.

(laughing) - Yeah, but nevertheless, that's what everyone calls him, so here we are. So that's a simplification, that's what a physicist would do. You and Richard Feynman sat down and talked about associative memory. Now if you look at the mind, where you can't quite simplify it so perfectly, do you think-- - Let me backtrack just a little bit.

- Yeah. - Biology is about dynamical systems. Computers are dynamical systems. You can ask, if you want to model biology, if you want to model neurobiology, what is the time scale? There's a dynamical system, in which, fairly fast time scale, in which you can say, the synapses don't change much during this computation, so I'll think of the synapses as fixed, and just do the dynamics of the activity.

Or you can say, the synapses are changing fast enough that I have to have the synaptic dynamics working at the same time as the system dynamics, in order to understand the biology. Most, if you look at the feedforward artificial neural nets, they're all done as learning, first of all, I spend some time learning, not performing, then I turn off learning and I perform.

- Right. - That's not biology. And so, as I look more deeply at neurobiology, even as an associate of memory, I've got to face the fact that the dynamics of a synapse change is going on all the time. And I can't just get by by saying, I'll do the dynamics of activity with fixed synapses.

- So the synaptic, the dynamics of the synapses is actually fundamental to the whole system. - Yeah, yeah. And there's nothing necessarily separating the time scales. When the time scales can be separated, it's neat from the physicist's or the mathematician's point of view, but it's not necessarily true in neurobiology.

- So you're kind of dancing beautifully between showing a lot of respect to physics, and then also saying that physics cannot quite reach the complexity of biology. So where do you land? Or do you continuously dance between the two points? - I continuously dance between them because my whole notion of understanding is that you can describe to somebody else how something works in ways which are honest and believable and still not describe all the nuts and bolts in detail.

Weather. I can describe weather as 10 to the 32 molecules colliding in the atmosphere. I can simulate weather that way, I have a big enough machine, I'll simulate it accurately. It's no good for understanding. If I just want to understand things, I want to understand things in terms of wind patterns, hurricanes, pressure differentials, and so on.

All things as they're collective. And the physicist in me always hopes that biology will have some things which can be said about it which are both true and for which you don't need all the molecular details of the molecules colliding. That's what I mean from the roots of physics.

My understanding. - So what did, again, sorry, but Hopfield Networks help you understand, what insight did it give us about memory, about learning? - They didn't give insights about learning. They gave insights about how things having learned could be expressed. How having learned a picture of you reminds me of your name.

That would, it didn't describe a reasonable way of actually doing the learning. Or at least that if you had previously learned the connections of this kind of pattern, would now be able to behave in a physical way which is a, ah, if I put part of the pattern in here, the other part of the pattern will complete over here.

I could understand that physics if the right learning stuff had already been put in. And it could understand why then putting in a picture of somebody else would generate something else over here. But it did not have a reasonable description of the learning process. - But even, so forget learning.

I mean, that's just a powerful concept that sort of forming representations that are useful to be robust, for error correction kind of thing. So this is kind of what the biology does we're talking about. - Yeah, and what my paper did was simply enable you, there are lots of ways of being robust.

If you think of a dynamical system, you think of a system where a path is going on in time. And if you think of a computer, there's a computational path, which is going on in a huge dimensional space of ones and zeros. And an error correcting system is a system which if you get a little bit off that trajectory, will push you back onto that trajectory again.

So you get to the same answer in spite of the fact that there were things, the computation wasn't being ideally done all the way along the line. And there are lots of models for error correction. But one of the models for error correction is to say, there's a valley that you're following, flowing down.

And if you push a little bit off the valley, just like water being pushed a little bit by a rock, gets back and follows the course of the river. And that basically the analog in the physical system, which enables you to say, oh yes, error free computation and an associative memory are very much like things that I can understand from the point of view of a physical system.

The physical system can be under some circumstances, an accurate metaphor. It's not the only metaphor. There are error correction schemes, which don't have a valley and energy behind them. But those are error correction schemes which a mathematician may be able to understand, but I don't. - So there's the physical metaphor that seems to work here.

- That's right, that's right. - So these kinds of networks actually led to a lot of the work that is going on now in neural networks, artificial neural networks. So the follow on work with restricted Boltzmann machines and deep belief nets followed on from these ideas of the Hopfield network.

So what do you think about this continued progress of that work towards now re-revigorated exploration of feed forward neural networks and recurrent neural networks and convolutional neural networks and kinds of networks that are helping solve image recognition, natural language processing, all that kind of stuff. - It's always intrigued me that one of the most long lived of the learning systems is the Boltzmann machine, which is intrinsically a feedback network.

And with the brilliance of Hinton and Sanofsky to understand how to do learning in that. And it's still a useful way to understand learning and understand, and the learning that you understand in that has something to do with the way that feed forward systems work. But it's not always exactly simple to express that intuition.

But it always amuses me to see Hinton going back to the will yet again on a form of the Boltzmann machine, because really that which has feedback and interesting probabilities in it is a lovely encapsulation of something computational. - Something computational? - Something both computational and physical. Computational in the, it's very much related to feed forward networks.

Physical in that Boltzmann machine learning is really learning a set of parameters for a physics Hamiltonian or energy function. - Mm-hmm. What do you think about learning in this whole domain? Do you think the aforementioned guy, Jeff Hinton, all the work there with back propagation, all the kind of learning that goes on in these networks, how do you, if we compare it to learning in the brain, for example, is there echoes of the same kind of power that back propagation reveals about these kinds of recurrent networks?

Or is it something fundamentally different going on in the brain? - I don't think the brain is as deep as the deepest networks go, the deepest computer science networks. And I do wonder whether part of that depth of the computer science networks is necessitated by the fact that the only learning that's easily done on a machine is feed forward.

And so there's the question of to what extent has the biology, which has some feed forward and some feed back, been captured by something which has got many more neurons, much more depth than neurons. - So part of you wonders if the feedback is actually more essential than the number of neurons or the depth, the dynamics of the feedback.

- The dynamics of the feedback. Look, if you don't have feedback, it's a little bit like building a big computer and running it through one clock cycle. And then you can't do anything 'cause you'd reload something coming in. How do you use the fact that there are multiple clocks?

How do I use the fact that you can close your eyes, stop listening to me and think about a chess board for a few minutes without any input whatsoever? - Yeah, that memory thing, that's fundamentally a feedback kind of mechanism. You're going back to something. - Yes. It's hard to understand.

It's hard to introspect, let alone consciousness. 'Cause that's all- - Let alone consciousness, yes, yes. - 'Cause that's tied up in there too. You can't just put that on another shelf. - Every once in a while, I get interested in consciousness and then I go and I've done that for years and ask one of my bettors, as it were, their view on consciousness.

And it's been interesting collecting them. - What is consciousness? Let's try to take a brief step into that room. - Well, I asked Marvin Minsky, the view on consciousness. And Marvin said, consciousness is basically overrated. It may be an epiphenomenon. After all, all the things your brain does, which are actually hard computations, you do non-consciously.

And there's so much evidence that even the simple things you do, you can make decisions, you can make committed decisions about them. The neurobiologist can say, he's now committed. He's going to move the hand left before you know it. - So his view that consciousness is not, that's just like little icing on the cake.

The real cake is in the subconscious. - Yeah, yeah. Subconscious, non-conscious. - Non-conscious, what's the better word, sir? - It's only that Freud captured the other word. - Yeah, it's a confusing word, subconscious. - Nicholas Chater wrote an interesting book. I think the title of it is "The Mind is Flat." (chuckles) Flat in a neural net sense, might be flat is something which is a very broad neural net without really any layers in depth, or the deep brain would be many layers and not so broad.

In the same sense that if you push Minsky hard enough, he would probably have said, consciousness is your effort to explain to yourself that which you have already done. (chuckles) - Yeah, it's the weaving of the narrative around the things that already been computed for you. - That's right, and so much of what we do for our memories of events, for example, if there's some traumatic event you witness, you will have a few facts about it correctly done.

If somebody asks you about it, you will weave a narrative, which is actually much more rich in detail than that, based on some anchor points you have of correct things, and pulling together general knowledge on the other, but you will have a narrative. And once you generate that narrative, you are very likely to repeat that narrative and claim that all the things you have in it are actually the correct things.

There was a marvelous example of that in the Watergate/impeachment era of John Dean. John Dean, you're too young to know, had been the personal lawyer of Nixon. And so John Dean was involved in the cover up, and John Dean ultimately realized the only way to keep himself out of jail for a long time was actually to tell some of the truths about Nixon.

And John Dean was a tremendous witness. He would remember these conversations in great detail, and very convincing detail. And long afterward, some of the tapes, the secret tapes, as it were, from which John Dean was recalling these conversations were published. And one found out that John Dean had a good, but not exceptional memory.

What he had was an ability to paint vividly, and in some sense accurately, the tone of what was going on. - By the way, that's a beautiful description of consciousness. (laughs) Do you, like where do you stand in your, today, (laughs) so perhaps it changes day to day, but where do you stand on the importance of consciousness in our whole big mess of cognition?

Is it just a little narrative maker, or is it actually fundamental to intelligence? - That's a very hard one. But I asked Francis Crick about consciousness. He launched forward in a long monologue about Mendel and the peas. - Yeah. - And how Mendel knew that there was something, and how biologists understood that there was something in inheritance, which was just very, very different.

And the fact that inherited traits didn't just wash out into a gray, but were this or this, and propagated, that that was absolutely fundamental to biology. And it took generations of biologists to understand that there was genetics, and it took another generation or two to understand that genetics came from DNA.

But very shortly after Mendel, thinking biologists did realize that there was a deep problem about inheritance. And Francis would have liked to have said, "And that's why I'm working on consciousness." But of course, he didn't have any smoking gun in the sense of Mendel. And that's the weakness of his position.

If you read his book, which he wrote with Koch, I think. - Yeah, Christoph Koch, yeah. - I find it unconvincing for the smoking gun reason. (sighs) So I've gone on collecting views without actually having taken a very strong one myself, because I haven't seen the entry point. Not seeing the smoking gun from the point of view of physics, I don't see the entry point.

Whereas in neurobiology, once I understood the idea of a collective, an evolution of dynamics, which could be described as a collective phenomenon, I thought, "Ah, there's a point "where what I know about physics "is so different from any neurobiologist "that I have something that I might be able to contribute." - And right now, there's no way to grasp a consciousness from a physics perspective.

- From my point of view, that's correct. And of course, people, physicists like everybody else, think very muddily about things. You ask the closely related question about free will, do you believe you have free will? Physicists will give an offhand answer and then backtrack, backtrack, backtrack, where they realize that the answer they gave must fundamentally contradict the laws of physics.

- Naturally, answering questions of free will and consciousness naturally lead to contradictions from a physics perspective. 'Cause it eventually ends up with quantum mechanics, and then you get into that whole mess of trying to understand how much, from a physics perspective, how much is determined, already predetermined, much is already deterministic about our universe.

There's lots of different-- - And if you don't push quite that far, you can say essentially all of neurobiology, which is relevant, can be captured by classical equations of motion. Because in my view of the mysteries of the brain are not the mysteries of quantum mechanics, but the mysteries of what can happen when you have a dynamical system, driven system with 10 to the 14 parts.

That that complexity is something which is, that the physics of complex systems is at least as badly understood as the physics of phase coherence in quantum mechanics. - Can we go there for a second? You've talked about attractor networks, and just maybe you could say what are attractor networks, and more broadly, what are interesting network dynamics that emerge in these or other complex systems?

- You have to be willing to think in a huge number of dimensions, 'cause in a huge number of dimensions, the behavior of a system can be thought of as just the motion of a point over time in this huge number of dimensions. - Right. - An attractor network is simply a network where there is a line, and other lines converge on it in time.

That's the essence of an attractor network. That's how you-- - In a highly dimensional space. - And the easiest way to get that is to do it in a high dimensional space, where some of the dimensions provide the dissipation, which means, which, look, I have a physical system, trajectories can't contract everywhere.

They have to contract in some places and expand in others. There's a fundamental classical theorem of statistical mechanics, which goes under the name of Liouville's theorem, which says you can't contract everywhere. You have to, if you contract somewhere, you expand somewhere else. And it's an interesting physical systems. You get driven systems where you have a small subsystem, which is the interesting part, and the rest of the contraction and expansion, the physicists would say is entropy flow in this other part of the system.

But basically, attractor networks are dynamics funneling down so you can't be any, so that if you start somewhere in the dynamical system, you will soon find yourself on a pretty well determined pathway, which goes somewhere. You start somewhere else, you'll wind up on a different pathway, but you don't have just all possible things.

You have some defined pathways, which are allowed and under which you will converge. And that's the way you make a stable computer. And that's the way you make a stable behavior. - So in general, looking at the physics of the emergent stability in these networks, what are some interesting characteristics that, what are some interesting insights from studying the dynamics of such high dimensional systems?

- Most dynamical systems, most driven dynamical systems, by driven they're coupled somehow to an energy source. And so if their dynamics keeps going because it's coupling to the energy source, most of them, it's very difficult to understand at all what the dynamical behavior is going to be. - You have to run it.

- You have to run it. There's a subset of systems, which has what is actually known to the mathematicians as a Lyapunov function. And those systems, you can understand convergent dynamics by saying you're going downhill on something or other. And that's what I found with ever knowing what Lyapunov functions were in the simple model I made in the early 80s, was an energy function.

So you could understand how you could get this channeling on the pathways without having to follow the dynamics in infinite detail. You started rolling a ball at the top of a mountain, it's going to wind up at the bottom of a valley. You know that's true without actually watching the ball roll down.

- There are certain properties of the system that when you can know that. - That's right. And not all systems behave that way. - Most don't probably. - Most don't, but it provides you with a metaphor for thinking about systems, which are stable and the good to have these attractors behave even if you can't find a Lyapunov function behind them or an energy function behind them.

It gives you a metaphor for thought. - Speaking of thought, if I had a glint in my eye with excitement and said, you know, I'm really excited about this, something called deep learning and neural networks, and I would like to create an intelligent system and came to you as an advisor, what would you recommend?

Is it a hopeless pursuit to use neural networks to achieve thought? Is it, what kind of mechanisms should we explore? What kind of ideas should we explore? - Well, you look at the simple networks, one-pass networks. They don't support multiple hypotheses very well. As I have tried to work with very simple systems, which do something which you might consider to be thinking, thought has to do with the ability to do mental exploration before you make it take a physical action.

- Almost a, like we were mentioning, playing chess, visualizing, simulating inside your head, different outcomes. - Yeah, yeah. And now you could do that in a feed-forward network because you've pre-calculated all kinds of things. But I think the way neurobiology does it, it hasn't pre-calculated everything. It actually has parts of a dynamical system in which you're doing exploration in a way which is.

- There's a creative element. Like there's an. - There's a creative element. And in a simple-minded neural net, you have a constellation of instances from which you've learned. And if you are within that space, if a new question is a question within this space, you can actually rely on that system pretty well to come up with a good suggestion for what to do.

If on the other hand, the query comes from outside the space, you have no way of knowing how the system's gonna behave. There are no limitations on what could happen. And so the artificial neural net world is always very much, I have a population of examples. The test set must be drawn from the equivalent population.

If the test set has examples which are from a population which is completely different, there's no way that you could expect to get the answer right. - Yeah, what they call outside the distribution. - That's right, that's right. And so if you see a ball rolling across the street at dusk, if that wasn't in your training set, the idea that a child may be coming close behind that is not going to occur to the neural net.

- And it is to our, there's something in the neurobiology that allows that. - Yeah, there's something in the way of what it means to be outside of the population of the training set. The population of the training set isn't just sort of this set of examples. There's more to it than that.

And it gets back to my question of what is it to understand something? - Yeah. You know, in a small tangent, you've talked about the value of thinking, of deductive reasoning in science versus large data collection. So sort of thinking about the problem. I suppose it's the physics side of you of going back to first principles and thinking, but what do you think is the value of deductive reasoning in the scientific process?

- Well, look, there are obviously scientific questions in which the route to the answer to it comes through the analysis of one hell of a lot of data. - Right. Cosmology, that kind of stuff. - And that's never been the kind of problem in which I've had any particular insight.

Though I must say, if you look at, cosmology is one of those. If you look at the actual things that Jim Peebles, one of this year's Nobel Prize in physics, one from the local physics department, the kinds of things he's done, he's never crunched large data. Never, never, never.

He's used the encapsulation of the work of others in this regard. - But ultimately boiled down to thinking through the problem. Like what are the principles under which a particular phenomenon operates? - Yeah, yeah. And look, physics is always going to look for ways in which you can describe the system in a way which rises above the details.

And to the hard-dyed-in-the-wool biologist, biology works because of the details. In physics, to the physicists, we want an explanation which is right in spite of the details. And there will be questions which we cannot answer as physicists because the answer cannot be found that way. (Peebles sniffs) - There's, I'm not sure if you're familiar with the entire field of brain-computer interfaces that's become more and more intensely researched and developed recently, especially with companies like Neuralink with Elon Musk.

- Yeah, I know there have always been the interest both in things like getting the eyes to be able to control things or getting the thought patterns to be able to move what had been a connected limb which is now connected through a computer. - That's right. So in the case of Neuralink, they're doing a thousand-plus connections where they're able to do two-way, activate and read spikes, neural spikes.

Do you have hope for that kind of computer-brain interaction in the near or maybe even far future of being able to expand the ability of the mind of cognition or understand the mind? - It's interesting watching things go. When I first became interested in neurobiology, most of the practitioners thought you would be able to understand neurobiology by techniques which allowed you to record only one cell at a time.

- One cell, yeah. - People like David Hubble very strongly reflected that point of view. And that's been taken over by a generation, a couple of generations later, by a set of people who says, "Not until we can record from 10 to the four "or 10 to the five at a time "will we actually be able to understand "how the brain actually works." And in a general sense, I think that's right.

You have to look, you have to begin to be able to look for the collective modes, collective operations of things. It doesn't rely on this action potential of that cell. It relies on the collective properties of this set of cells connected with this kind of patterns and so on.

And you're not going to succeed in seeing what those collective activities are without recording many cells at once. - The question is how many at once? What's the threshold? And that's the-- - Yeah, and look, it's being pursued hard in the motor cortex. The motor cortex does something which is complex, and yet the problem you're trying to address is fairly simple.

Neurobiology does it in ways that are different from the way an engineer would do it. An engineer would put in six highly accurate stepping motors controlling a limb rather than 100,000 muscle fibers, each of which has to be individually controlled. And so understanding how to do things in a way which is much more forgiving and much more neural, I think, would benefit the engineering world.

The engineering world, ah, touch. Let's put in a pressure sensor or two, rather than an array of a gazillion pressure sensors, none of which are accurate, all of which are perpetually recalibrating themselves. - So you're saying your hope is, your advice for the engineers of the future is to embrace the large chaos of a messy, error-prone system like those of the biological systems.

Like that's probably the way to solve some of these. - I think you'll be able to make better computations slash robotics that way than by trying to force things into a robotics where joint motors are powerful and stepping motors are accurate. - But then the physicists, the physicists in you will be lost forever in such systems 'cause there's no simple fundamentals to explore in systems that are so large and messy.

- Well, you say that, and yet there's a lot of physics, the Navier-Stokes equations, the equations of nonlinear hydrodynamics, huge amount of physics in them. All the physics of atoms and molecules has been lost, but it's been replaced by this other set of equations, which is just as true as the equations at the bottom.

Now those equations are going to be harder to find in general biology, but the physicist in me says there are probably some equations of that sort. - They're out there. - They're out there, and if physics is going to contribute to anything, it may contribute to trying to find out what those equations are and how to capture them from the biology.

- Would you say that's one of the main open problems of our age is to discover those equations? - Yeah, if you look at, there's molecules and there's psychological behavior, and these two are somehow related. They're layers of detail, they're layers of collectiveness, and to capture that in some vague way, several stages on the way up to see how these things can actually be linked together.

- So it seems in our universe, there's a lot of elegant equations that can describe the fundamental way that things behave, which is a surprise. It's compressible into equations. It's simple and beautiful, but it's still an open question whether that link is equally between molecules and the brain is equally compressible into elegant equations.

But your sense, you're both a physicist and a dreamer. You have a sense that-- - Yeah, but I can only dream physics dreams. - You can only dream physics dreams. - There was an interesting book called "Einstein's Dreams," which alternates between chapters on his life and descriptions of the way time might have been, but isn't.

The linking between these being, of course, ideas that Einstein might have had to think about the essence of time as he was thinking about time. So speaking of the essence of time and your biology, you're one human, famous impactful human, but just one human with a brain living the human condition, but you're ultimately mortal, just like all of us.

Has studying the mind as a mechanism changed the way you think about your own mortality? - It has really, because particularly as you get older and the body comes apart in various ways, I became much more aware of the fact that what is somebody is contained in the brain and not in the body that you worry about burying.

And it is to a certain extent true that for people who write things down, equations, dreams, notepads, diaries, fractions of their thought does continue to live after they're dead and gone, after their body is dead and gone. And there's a sea change in that going on in my lifetime between when my father died, when except for the things which were actually written by him as it were, very few facts about him will have ever been recorded.

And the number of facts which are recorded about each and every one of us forever now, as far as I can see in the digital world. And so the whole question of what is death may be different for people a generation ago than a generation further ahead. - Maybe we have become immortal under some definitions.

- Yeah, yeah. - Last easy question, what is the meaning of life? Looking back, you've studied the mind, us weird descendants of apes, what's the meaning of our existence on this little earth? - Oh, that word meaning is as slippery as the word understand. - Interconnected somehow perhaps. Is there, it's slippery, but is there something that you, despite being slippery, can hold long enough to express?

- Well, I've been amazed at how hard it is to define the things in a living system in the sense that one hydrogen atom is pretty much like another, but one bacterium is not so much like another bacterium, even of the same nominal species. In fact, the whole notion of what is the species gets a little bit fuzzy.

And the species exists in the absence of certain classes of environments. And pretty soon one winds up with a biology which the whole thing is living, but whether there's actually any element of it, which by itself would be said to be living, it becomes a little bit vague in my mind.

- So in a sense, the idea of meaning is something that's possessed by an individual, like a conscious creature. And you're saying that it's all interconnected in some kind of way that there might not even be an individual, or all kind of this complicated mess of biological systems at all different levels where the human starts and when the human ends is unclear.

- Yeah, yeah, and we're in neurobiology where the, oh, you say the neocortex is at the thinking, but there's lots of things that are done in the spinal cord. And so we say, what is the essence of thought? Is it just gonna be neocortex? Can't be, can't be. - Yeah, maybe to understand and to build thought, you have to build the universe along with the neocortex.

It's all interlinked through the spinal cord. John, it's a huge honor talking today. Thank you so much for your time. I really appreciate it. - Well, thank you for the challenge of talking with you. And it'll be interesting to see whether you can win five minutes out of this with just coherent sense to anyone or not.

- Beautiful. Thanks for listening to this conversation with John Hopfield, and thank you to our presenting sponsor, Cash App. Download it, use code LEXPODCAST, you'll get $10, and $10 will go to FIRST, an organization that inspires and educates young minds to become science and technology innovators of tomorrow. If you enjoy this podcast, subscribe on YouTube, get five stars on Apple Podcasts, support on Patreon, or simply connect with me on Twitter @LexFriedman.

And now let me leave you with some words of wisdom from John Hopfield in his article titled, "Now What?" Choosing problems is the primary determinant of what one accomplishes in science. I have generally had a relatively short attention span in science problems. Thus, I have always been on the lookout for more interesting questions, either as my present ones get worked out, or as they get classified by me as intractable, given my particular talents.

He then goes on to say, "What I have done in science "relies entirely on experimental and theoretical studies "by experts. "I have a great respect for them, "especially for those who are willing to attempt "communication with someone "who is not an expert in the field." I would only add that experts are good at answering questions.

If you're brash enough, ask your own. Don't worry too much about how you found them. Thank you for listening, and hope to see you next time. (upbeat music) (upbeat music)