Before, action, after

Watch a baby in a high chair with a spoon. The spoon gets lifted, studied, and dropped over the side. Clatter. An adult picks it up and returns it. It gets dropped again. Clatter. By the fifth drop the adult has theories about the baby’s motives, and none of them are charitable.

Here’s what the baby actually has after five drops: five records that all share the same three parts. What things were like. What I did. What things were like next. The spoon was in my hand, I opened my fingers, and then my hand was empty and a bang came from below. Before, action, after. The baby is running experiments, and every experiment produces a record of exactly this shape.

That shape is the foundation this entire book rests on, so let me give it its name. A triplet is one recorded moment of experience: what I sensed, what I did, and what I sensed next. Everything PRA will ever learn, it learns from a stream of triplets. There is no other input. No textbook, no labels, no instructions. Those would be someone else’s knowledge. The triplet stream is the machine’s own life, and my claim is that it’s enough.

One triplet, in numbers

These words need pinning down before we go further, because the whole book stands on them and “sensed” is doing a lot of work.

An observation is one simultaneous reading of every sensor the body has, packed into a fixed-order list of numbers. Nothing more. The rover you’ll meet in Part 4 has five distance rays, a two-number compass, a two-number position beacon, and a bump detector, so one of its observations is ten numbers, always in that order. And the “before” and “after” of a triplet are both simply observations: the same ten slots, read at two consecutive steps. The triplet is the package of three things: the reading before, the action taken, the reading after.

Here is one, from the rover, with the noise stripped for readability:

before:  rays 0.61 0.44 0.30 0.51 0.72   compass 0.00 1.00   position 0.05 -0.32   bump 0
action:  forward
after:   rays 0.54 0.37 0.22 0.44 0.65   compass 0.00 1.00   position 0.05 -0.24   bump 0

The labels are for you. The brain receives ten unlabeled numbers, then an action, then ten more. Look at what moving forward did: the five ray numbers all shrank (something ahead is getting closer), the compass numbers held still (no turning happened), one position number grew by the size of a step, and the bump flag stayed quiet. One triplet like this proves nothing. But collect thousands, and the regularity is sitting there in plain arithmetic: whenever the action was “forward”, the rays shrink together and the compass holds; whenever it was “turn_left”, the rays reshuffle and the compass moves instead. Nobody tells the brain that the first five slots are distances, or that walls exist. Cause and effect is lying in the differences between before and after, waiting to be mined. That mining is Part 3’s job.

Why the middle part changes everything

Strike out the middle of the triplet and you’re left with: what I sensed, then what I sensed next. Just watching. A lot can be learned by watching. You notice which sights tend to follow which. But there’s a wall that watching can never get through.

Every morning the rooster crows, and then the sun comes up. A pure watcher sees these two events go together thousands of times without a single exception. Does the crowing pull the sun up? To a watcher, “the crow causes the dawn” and “the crow merely comes first” look identical. They are identical in the watching. The only way to tell them apart is to reach into the world and meddle. Keep the rooster quiet for one morning and see whether the sun still rises.

That’s what the middle of the triplet is. An action is a deliberate poke at the world, and the “after” is the world’s reply to your poke rather than to the general flow of events. I did this, and then that happened, and I know the “this” was mine. Stack up enough of those records and you learn something that watching can’t deliver at any volume: what your actions actually do. Cause and effect can only be observed from one vantage point, and that’s the inside of something that acts.

This is why I keep insisting the book isn’t about language or vision. Those are kinds of content, and the triplet is about the structure of experience: it comes as before, act, after, and the middle belongs to you. Any content fits the shape. A rover’s triplet holds laser ranges and wheel commands. A robot hand’s holds finger pressures and motor currents. And a conversation fits, as chapter 1 promised: I heard something, I said something, something came back. The shape doesn’t care what flows through it, and that indifference is what will let one mechanism serve every body in this book.

The test that needs no teacher

So a brain eats triplets. How do we know whether it’s learning anything? The shape carries its own test, and the test is cheap: prediction. If you understand what your actions do, you can say the “after” before it arrives. My fingers are about to open. What happens next?

The world then does something no teacher could be paid enough to do. It grades every prediction instantly, and it has no interest in your feelings. The spoon lands where it lands. The gap between what you predicted and what arrived is your surprise, and it comes delivered fresh with every triplet, thousands of times a day, for free. A big surprise means your model of the world is wrong right here. No surprise means there’s nothing left to learn in this corner today.

Remember the crowd of small rival models this book promised you early on?¹ This is the game they compete at: smallest surprise on the next triplet. Every mechanism in Part 3 runs on prediction error as its fuel. Survival, eviction, even the discovery of size.

Under the hood: the event contract. In PRA-01 the triplet is the SensorimotorEvent(previous_observation, action, observation), and its §3.1 ordering requirement is load-bearing: previous_observation is always the true observation from the immediately preceding step, regardless of what any frame chose to attend to or map. A frame that ignored the last step still receives the real history in its next event. The rule exists because the alternative is chapter 2’s disease in a new organ: a model allowed to curate its own experience record is grading its own homework. History is written by the world, once, for everyone.

What’s not in a triplet

Look once more at the shape: sensed, did, sensed. Now notice what’s missing. I left it out on purpose, and it’s the most opinionated design choice in the book so far.

There is no score in it. No “that was good.” No reward.

Most learning machines that act are built around a reward signal. Game-playing systems are the famous example: a number arrives with each step, the number says how well you’re doing, and the machine exists to make the number big. PRA’s triplets carry no such number. When a game world is plugged into PRA, the game’s built-in reward is deliberately left at the door. The brain receives sights and sends actions, and that is the whole interface.

Why refuse free information? Because a reward is somebody’s opinion baked into a number. The game says the score is what matters. A warehouse robot’s designers say boxes per hour is what matters. Whoever picks the reward has decided, in advance, what the machine is for, and that decision rides along inside every step of training. Chapter 1 all over again, one level deeper.

So PRA splits the job in two. The triplet stream teaches the brain how the world works: do this, and that happens. Motivation, the business of deciding which action is worth taking, is a separate component with its own chapter (chapter 9). Keeping the two apart has a practical payoff: you can change the machine’s job without touching its understanding of the world. What a motor command does stays true whether this week’s task is sorting packages or stacking shelves. A brain that mixed the two would have to relearn its world every time its job changed, and a brain that learns forever will change jobs many times.

One spoon-drop proves nothing, of course. Maybe the clatter was a coincidence. Maybe the floor won’t be there next time. A single triplet is one data point from one poke at an enormous world. The power was never in the triplet. It’s in the stream, millions of triplets over a lifetime, and in the kind of machinery that can eat that stream and come out understanding. Before we open that machine, though, there’s an objection standing in the doorway: machines that seem to understand already exist, and you can talk to one today. The next chapter is about why this book isn’t about them.

1. Chapter 2. ↩