I've found the most practical rule is: "Would I want to be paged for this at 2 AM?"

If yes: ERROR If I want to check it tomorrow: WARNING If it's useful for debugging: INFO Everything else: DEBUG

The problem with the article's approach is that libraries don't have enough context. A timeout calling an external API might be totally fine if you're retrying, but it's an ERROR if you've exhausted retries and failed the user's request.

We solve this by having libraries emit structured events with severity hints, then the application layer decides the final log level based on business impact. A 500 from a recommendation service? Warning. A 500 from the payment processor? Error.