No - imperative systems can do that too. Take Chef (an imperative tool) for example - it can trivially examine a tracked file and mutate the state if the state

No - imperative systems can do that too. Take Chef (an imperative tool) for example - it can trivially examine a tracked file and mutate the state if the state is not correct. This satisfies the "diff" criteria you mentioned above. Isn't Chef looking sort of declarative? The nuance is subtle but important. Let's say you stop tracking that file. In Chef, the file will simply remain in its last known state, because Chef forgot about it. Likewise, if you do the same process on other systems, files will be left in various states - causing configuration drift. This is what a declarative tool is meant to solve. A proper declarative tool would _delete_ the file, because it's no longer tracked, to ensure that the system is in the state the user expects.