---
slug: "BayesUpdate"
name: "BayesUpdate"
type: "Inference"
layer: "Mind"
ref: "BayesUpdate#8656"
sema_id: "sema:BayesUpdate#mh:SHA-256:865637baf030de8ee9babd2a2fdddbf68d99353516aa86eef07cf6bc1e8a5d38"
tags: ["Mind","Inference"]
updated: 2026-11-11
mindscape: "bootstrap"
index_url: https://mindscape.pages.dev/bootstrap/index.json
url: https://mindscape.pages.dev/bootstrap/BayesUpdate/
related: ["BaseRateInclude","Belief","Observe","ConfidenceCalibrate","HypothesisLadder"]
---

# BayesUpdate

Mathematically rigorous belief revision

Layer: Mind · Category: Inference

## Mechanism

Likelihood Weighting: Hold prior probability P(H). [Observe](https://mindscape.pages.dev/bootstrap/Observe.md) evidence E. Compute likelihood ratio P(E|H)/P(E|¬H). Convert the prior to odds, multiply by the ratio, convert back to a probability. Large ratio = strong update. Ratio near 1 = weak evidence. Never update to 0 or 1. It explicitly incorporates [BaseRateInclude](https://mindscape.pages.dev/bootstrap/BaseRateInclude.md) into the prior probability calculation to prevent base-rate neglect.

## Invariants

- The posterior odds are the prior odds scaled by the likelihood ratio P(E|H)/P(E|not H).
- Prior and posterior both stay at least `clamp` away from 0 and 1.

## Preconditions

- Prior probability defined in (0,1)
- New evidence observed.
- Likelihood computable.

## Postconditions

- Posterior probability in (0,1)
- [Belief](https://mindscape.pages.dev/bootstrap/Belief.md) updated proportional to evidence strength.
- Prior recoverable from the posterior and the ratio, unless the posterior was clamped.

## Failure modes

- Prior Dogmatism: Setting a prior of 1.0 or 0.0 prevents any future updating, regardless of evidence.

## Parameters

- `clamp` (Probability, (0.0, 0.5)): Distance from 0 and 1 that no probability may cross, so evidence can always move it.

## Builds on

- [BaseRateInclude](https://mindscape.pages.dev/bootstrap/BaseRateInclude.md) (Mind; References)
- [Belief](https://mindscape.pages.dev/bootstrap/Belief.md) (Infrastructure; References)
- [Observe](https://mindscape.pages.dev/bootstrap/Observe.md) (Infrastructure; References)

## Used by

- [ConfidenceCalibrate](https://mindscape.pages.dev/bootstrap/ConfidenceCalibrate.md) (Mind; References)
- [HypothesisLadder](https://mindscape.pages.dev/bootstrap/HypothesisLadder.md) (Mind; References)

## Identity

- Sema ID: `sema:BayesUpdate#mh:SHA-256:865637baf030de8ee9babd2a2fdddbf68d99353516aa86eef07cf6bc1e8a5d38`
- Ref: `BayesUpdate#8656`
- Supersedes:
  - `sema:BayesUpdate#mh:SHA-256:911b3e915019f09fbbbde387283af064db0903ce67a7219b0f27730c236de487`
  - `sema:BayesUpdate#mh:SHA-256:3d1ba13d30f25ea87acd410653f8e4477e280d0f5a204cd1eacf96de4013f7db`
  - `sema:BayesUpdate#mh:SHA-256:879afc8b39356ead46edabbc3f4f098d9e7e5995a516d573574427dcb669fa3a`
  - `sema:BayesUpdate#mh:SHA-256:11d261e6588f2923517c746a910aa2a60e6f3998852e109ea533d7199ecd4db8`

## Links

- [Pattern page](https://mindscape.pages.dev/bootstrap/BayesUpdate/)
- [View in the interactive graph](https://mindscape.pages.dev/bootstrap/#BayesUpdate)
- [JSON](https://mindscape.pages.dev/bootstrap/BayesUpdate.json)
- [Index of all patterns](https://mindscape.pages.dev/bootstrap/index.json)