research · pra · the body # The anatomy of a body. You come with one anatomy for life: brain, eyes, ears, hands, mouth. A PRA doesn't. Its body is whatever the world affords — formally, just two lists: sensors in, actuators out. **The brain is on neither list.** ### What is a world? Almost anything. The contract is three lines: PRA can **sense** a state as it changes; it can **act**, and acting changes what it senses next; and the results come back **soon enough to learn from**. A game qualifies. A robot's room qualifies. So does a market feed, a hand on an object, or attention moving through a space of ideas. The mechanism doesn't care which, and this page is about the piece that makes that indifference possible: the body. ### What does a body in that world look like? The body is the only part of PRA that touches the world, and it is smaller than the word suggests: a body is exactly **two lists**. A list of sensors, read in a fixed order into one observation. A list of acts it can perform, one per tick. Here is the body PRA wears in Minecraft today, grouped the way a doctor would group it: *[diagram: The blocky bot with its twelve sense channels called out on the left as eyes, balance, taste, gut, skin, pockets and fingertips, its thirteen acts on the right as mouth, hands, legs and idle — and an empty head: the brain is not a body part.]* figure 1 · the survival body, organ by organ Two things in that picture carry the whole idea. First, **the head is empty on purpose**. The brain — the frames, the drive, the event head — is not a body part. It plugs in at the seam, and it is the same engine whatever body it wears. Second, some organs are missing: this body has **no ears**, because sound is not one of its channels. A part of the world a body cannot sense does not exist for it — designing an anatomy is deciding what its world *is*. The mouth shows how far the no-meaning rule goes. It has no edibility check: "eat" is just *use what you hold*, and what that does is the world's to decide — nourishment reaches the brain only as the next observation. Taste isn't wired in either: the palate is a price book the body learns from its own meals, shown to the brain as *worth*, never as names. Ask for another world and you get another body: *[diagram: One engine box connects through a seam to five bodies drawn as strips of cells: CartPole with four, a rover with ten, a ROS2 robot with twelve, a sample-field probe with sixteen, and the Minecraft survival body with eighty-six.]* figure 2 · one engine, five bodies, one scale That the engine truly doesn't change is measured, not promised: a probe body sharing nothing structural with Minecraft ran on the same engine with zero code changes and passed every transfer bar. ### How does the brain talk to the body? Organ words are for us. The brain sees none of them. One observation is 86 numbers in a fixed order; one act is an index between 0 and 12. No names cross the seam, and no item classes: a thing appears only as properties the world itself asserts — placeable, edible, a count — and a small appearance signature. Categories are the brain's to form. *[diagram: A strip of 86 small squares grouped into twelve named sense channels flows into one engine box, which emits one of thirteen acts; an arrow labelled the world answers loops the acts back to the strip.]* figure 3 · the seam, and the only feedback there is There is no reward wire in that picture, and no success flag coming back from the acts. The only feedback the brain ever receives is **the next observation**. That is why one engine can run any body: it never needs to know what a number means, only that the ordering is stable and that acting changes what arrives next. Wire in a world of your own through that same seam — two lists, nothing more — and PRA starts learning it. 86 numbers in · 13 acts out · verified against the running body