Where Does the Why Live?

Question#

Where does "why" live?

Framing: "why" = rationale, one of the PRD's three jobs#

In this wiki, the why is rationale — the record of why a decision was made. It is one of the three jobs the PRD used to do at once (The PRD-Replacement Spectrum at AI-Native Speed):

Alignment — everyone holds the same idea (Brooks's design concept).
Specification — define what to build precisely enough to build it.
Rationale — record why the decisions were made, for future readers.

When agentic coding makes generation nearly free, specification dissolves and alignment relocates — but rationale is orphaned. The question "where does why live" is therefore really two questions, split by when you ask.

Going in (authoring time): the why is the protagonist#

At decision time the why has a deliberate, well-defended home. AI-native product methods are explicitly built so the human owns the why and hands the what to the model:

The recorded "why-not-what" conversation. Dan Carey's method (Prototype Over PRD) makes the rule load-bearing: "talk about why, not what" — why the problem matters and what characterizes a good solution, never the buttons/screens — then transcribe it and hand it to Claude for prototype variations. The why is the input; the what is generated.
The grilling session. Matt Pocock's grill-me (Design Concept Grilling) drives toward Brooks's design concept — a shared idea of why this, this way — before any artifact exists. The artifact at the end is "the conversation history itself."

So the entire right half of the PRD-replacement spectrum works precisely by making the why the human's job and letting model capability fill the what. Read this way, the why isn't lost — it's the alignment substrate everything downstream is generated from.

Coming out (for the next reader): the why is orphaned#

This is the genuinely unsolved part, and the wiki flags it independently in four places. Every position that dissolves the spec also throws away the document that used to carry the rationale:

The destination PRD that held it gets deleted after implementation (Design Concept Grilling, Prototype Over PRD).
Design discussion gets buried in merged PRs — Building Is Cheap, Arguing Is Expensive open question: "where is the rationale recorded for future readers — does the 'why we chose this' knowledge survive?"
The prototype shows the what, not why it beat alternative A — Prototype Over PRD open question: "If there is no PRD, where does the rationale live?"

Why each candidate home fails#

Candidate home	Verdict	Source
The code / repo	Explicitly excluded. Fung's own open question names "the 'why'" as knowledge that genuinely can't live in the codebase (alongside org strategy, cross-team context).	Code as Source of Truth
Context files (CLAUDE.md / AGENTS.md / SPEC.md)	Policy plane — captures how the agent should behave and process invariants, not why product decision B beat A. Excellent at conventions and role boundaries; poor at decision history.	Agent Context Files
The destination PRD	Held the why momentarily, then deleted after implementation — so the why dies with the doc.	Design Concept Grilling
Richer durable artifact / interactive HTML	The only partial answer on the table: a durable interactive artifact can carry rationale a deleted PRD cannot.	HTML as the New Markdown

The code exclusion is the sharpest point: Code as Source of Truth is the strongest "put everything in the repo" position in the wiki, and even it carves out the why as the thing that can't go there. So "just check it into the codebase" — the default AI-native answer for specs and process — is not available for rationale.

The implicit fifth answer: a compiled knowledge base#

One home the source pages don't name, but that this vault embodies: a compiled, current knowledge base (LLM-as-Compiler Knowledge Base) is exactly a durable home for the why that sits outside the code. Where the repo stales (which is why the why can't live there) and the PRD gets deleted, a compiled artifact is kept current by design and is dual-audience (human + LLM). This is the same instinct as the richer-artifact axis — a durable, navigable artifact carrying what the code and the deleted doc cannot — one level up. The ## Open Questions sections of concept pages are, in effect, a running ledger of orphaned whys; this very page is one of them being given a home.

Bottom line#

"Where does why live" has a dual answer:

Going in, the why is well-homed — it's the recorded why-not-what conversation (Prototype Over PRD) and the grilling session (Design Concept Grilling); AI-native methods make it the protagonist.
Coming out, the why is homeless — every spec-dissolving move (delete the PRD, discuss in PRs, ship the prototype) discards the carrier of rationale, the code explicitly can't hold it (Code as Source of Truth), and context files hold policy not product-rationale (Agent Context Files).

Cheap building dissolved specification and relocated alignment into the artifact — but it orphaned rationale. The wiki has no clean fix, only two partial patches: the richer durable artifact (HTML as the New Markdown) and the compiled knowledge base (LLM-as-Compiler Knowledge Base). Until one is adopted deliberately, the answer to "where does the why live?" for the next reader is, honestly: nowhere durable — by default it evaporates.

Connections#

The PRD-Replacement Spectrum at AI-Native Speed — parent synthesis; names "orphaned rationale" as the first of two unsolved debts. This page is the focused deep-dive on that debt.
Prototype Over PRD — authoring-time home (the recorded why-conversation); raises the orphaned-rationale open question
Design Concept Grilling — authoring-time home (the grilling session reaches the design concept); PRD deleted after implementation
Building Is Cheap, Arguing Is Expensive — design discussion buried in PRs; raises the orphaned-rationale open question
Code as Source of Truth — the candidate home that explicitly excludes the why
Agent Context Files — policy plane, not product-rationale plane
HTML as the New Markdown — the richer-artifact partial answer
LLM-as-Compiler Knowledge Base — the compiled-knowledge-base partial answer (what this vault is)

Sources#

The PRD-Replacement Spectrum at AI-Native Speed — the three PRD jobs; rationale orphaned as unsolved debt #1
Prototype Over PRD — Dan Carey, why-not-what conversation + rationale open question
Design Concept Grilling — Matt Pocock, grilling reaches the design concept; PRD deleted
Building Is Cheap, Arguing Is Expensive — Fiona Fung, discussion-in-PRs + rationale open question
Code as Source of Truth — Fiona Fung, the why explicitly can't live in the codebase
Agent Context Files — context files are the policy plane
HTML as the New Markdown — the richer-artifact axis
LLM-as-Compiler Knowledge Base — the compiled knowledge base as durable home