Rigorous discussion on lack of baked-in authentication

[Alexander Farley]

Has there been any rigorous discussion of why Rails doesn't have baked-in authentication? I've seen a handful of posts alleging that "authentication varies too much" but I'm having trouble envisioning which fundamentally distinct use-cases would prevent a single library from solving 98% of user needs. 

Can anyone point to such a discussion? Failing that, I'd like to hear about various use cases which would be incompatible or messy if they were implemented in a single library. 

Being an 'opinionated' framework, it seems like authentication is something that would benefit from an opinion. 

[James Davis, PhD]

If there were to be a 'backed-in' authentication solution, it would likely be devise or something very devise-ish. I think that would be a little too presumptuous of Rails to force a heavyweight solution upon users. There are existing application generators that bundle devise into a new app's creation. I do not see a need for what you ask.