[danbruc]

Is it even theoretically possible to show that markets are efficient on purely theoretical grounds? As a thought experiment, imagine that all human brains have a weird defect that causes them to always ignore some specific kind of information when making pricing decisions, including when they write software dealing with those decisions, and under all other conceivable circumstances. In that case, it seems to me, it would be impossible to decide on purely theoretical grounds that markets are efficient because it strictly depends on the empirical observation that human brains have this weird defect. I could imagine the other way around to work, i.e. that there could be an obstruction to markets being efficient that can be shown to exist on purely theoretical grounds. But no matter what the answer to the second question is, in no case could you arrive at an »if and only if« result without including the empirical observation that brains do not have this weird defect.