@EnterprisingA (Jon Ayre) suggests an #EAMantra "If you know it, add it to your architecture. If you think you know it, add that too."
My immediate objection to this rule was that it seems to turn the architecture into a kind of brain dump. A good architect knows many things, and if all these items of knowledge go indiscriminately into the architecture, the architecture gets rather cluttered.
@EnterprisingA 's first defence was to say that "the quality of a brain dump depends on the quality of the brain from which the dump comes". And of course only the architect gets to deposit knowledge into the architecture (otherwise there would be a free-for-all).
Unfortunately, the brilliance of the architect is no guarantee that the architect's thoughts all satisfy some meaningful criterion of quality. Intelligence doesn't necessarily mean always having better-quality knowledge, but it should mean having a better capability for processing and developing and filtering and revising knowledge. And as Dumbledore says, "Being - forgive me - rather cleverer than most men, my mistakes tend to be correspondingly huger".
So the architect's knowledge may well get into the architecture eventually, but it doesn't need to go straight in. But @EnterprisingA is (quite reasonably) worried about not writing stuff down. "Keeping it in your head until you are absolutely certain is a good way of never being certain (or being challenged)." "I'd like you to review the arch but it's not certain til it's reviewed so I haven't drawn it so you can't review it..." Of course that's true, but I don't agree that there are only two places to keep knowledge - either in your head or in the architecture. @rgechristie suggests a third possibility - an interim area for in progress work before it hits "the architecture" (that still needs to be accessible by all developers and consumers of the architecture).
My second objection was that the architect has a lot of knowledge that doesn't really belong in the architecture at all. There is a strong temptation to put too much into an architecture, and the architectural documents can easily get bloated with miscellaneous material that the architects imagine might be useful to developers and others. (Methodology documents are subject to the same tendency.) For example, the architect may review a design document, may spot a particularly egregious design flaw, and decide to add a rule into the architecture, so that the architecture evolves into a general-purpose design handbook. And if every pearl of wisdom spoken by the architect has to be preserved in "The Architecture", you end up with "The Baroque" - a highly decorated and convoluted style.
It is of course possible to slim down an architecture with too much detail, but in practice this doesn't happen as often as it should. It is much easier to add material to a document than to subtract it, so over time the documents get longer and harder to read.
Following our debate on Twitter, @EnterprisingA reformulates his mantra to specify architectural decisions only. "If you know it is the right architectural choice, add it to your architecture. If you think you know it is, add that too." Surely I can't disagree with the proposition that "architectural decisions should be in the architecture", can I?
Actually I do. Sometimes the best architectural decision is to leave something out of the architecture, to leave something deliberately open and unspecified, underdetermined rather than overdetermined. This is a principle of just-enough architecture, or Zen architecture. Sometimes less is more.