What is a why-log?
A record of decisions with the reasoning attached, so the thinking survives the decision.
Definition
why-log (n.) – a running record in which each decision is stored together with the reasoning that produced it, the alternatives that were rejected, and the conditions under which it should be revisited.
Companies record decisions constantly. They record them as outcomes: a price, a supplier, a policy, a tool. What almost nobody records is why, and the why is the part that determines whether the decision still holds when circumstances change.
Without it, a company relitigates the same questions on a cycle. Somebody proposes the thing that was already tried, nobody remembers why it was abandoned, and the experiment runs a second time at full cost. With it, the answer is a file rather than a memory, and the file states what would have to be true for the decision to change.
A why-log entry that only records what was decided is not a why-log entry. The rejected alternatives are load-bearing, because they are what a future reader needs in order to avoid repeating them.
Entries are appended, never rewritten. A why-log that gets edited to look consistent has destroyed the thing it exists to preserve.
What an entry contains
The decision itself. The reasoning that led to it, stated plainly enough that somebody who was not there can follow it. The alternatives considered and why each was set aside. The conditions that would make this decision wrong, which is the part that turns a record into something useful later.
The date matters more than it looks. A decision that was correct given what was known in March is not a mistake because the situation changed in September, and the log is what makes that distinction visible.
Why it is the keystone
Craft can be rewritten. Playbooks can be rebuilt by running the procedure again. The reasoning behind a decision cannot be reconstructed, because it depended on context that no longer exists. It is the one artifact that is genuinely lost when a person leaves, and the only one that has to be captured at the time.
This is the concrete form of the argument for keeping a company in files. The files are not a backup of what people know. They are the only copy of the part that was never written down anywhere else.