Who defines the policies?
The policies are determined by consensus: proposed new policies and changes are not approved until a decision that suits everyone or almost everyone is found. The role of a “policy forge” is largely assumed by Wookieepedia:Consensus track. A discussion may start as a simple vote or even a simple suggestion or question, but usually, the end result is neither of the originally proposed options (in case of a vote), but rather some kind of fusion of different ideas. The process is often painful, but necessary.
