I'm going to make the provocative argument that the "proof of brain" idea puts the brain on the wrong side of the ledger. Content creators should earn tokens for "drawing human attention", not for "using their brain". Why? Because the Steem blockchain is a blockchain play to draw value from the attention economy.

I think the idea behind "proof of brain" is to go beyond what people do pay attention to and more thoughtfully assess underlying value, analogous to the way a philosopher might encourage someone to go beyond their initial thoughts on an issue and instead engage in "reflective equilibrium" before settling on their views.

In the end, if no one reads an article, it doesn't matter how much brain went into producing it. The value of a particular piece of content derives from the "focused mental engagement" that it receives.

Theories of value (like the Labor Theory of Value, etc.) are tricky. It might be the case that intrinsic value isn't something that things have, or if they do that may not be accessible to us, we may be limited to determining what things seem to be worth to us (similar to the way we can't directly access information about the world, we have to infer and deduce based on what we perceive). Ideally we'd have institutions that help us do things like find consensus, avoid over- and under-valuing things, etc. (but obviously that's easier said than done).

If the ranking is random, then there's no incentive at all to post here. If the ranking is based purely on stakeholder investment, then there's nothing to distinguish STEEM from a proof of stake blockchain, and little reason for any non-investor to visit the web sites that display Steem content. If the ranking is based on some idea of "quality" that's not accompanied by reach, then growth is limited. To maximize Steem's value, the ideal ranking at payout time has to correlate with the total amount of attention that a piece of content receives.

I think this is somewhat bidirectional. To the extent that the ranking is not valuable people will not pay attention to it. People for the most part already know what they do pay attention to, so reflecting that back to them is probably not too valuable. Being able to correctly tell people what would be beneficial for them to pay attention to is potentially valuable, and people might reward that with their attention.

A simple example is for the content producer to include questions in their post and upvote to reward answers in their replies.

As an aside, I wonder if generating good questions about articles is a task that the current generation of AI would be good at.