Two economists – one young and one old – are walking down the street together:

The young economist looks down and sees a $20 bill on the street and says, “Hey, look a twenty-dollar bill!”

Without even looking, his older and wiser colleague replies, “Nonsense. If there had been a twenty-dollar lying on the street, someone would have already picked it up by now.”

(99% joke and snark. Of course this isn't going to work on the stock market, but that kind of efficiency argument isn't perfect.)

I was not making the efficiency argument in the abstract. I'm making it in the concrete of a neural network that can be trained in real time in a browser window.

There's a great deal of people who seem to mistake "the market isn't actually 100% efficient" for "oh, I guess we can just ignore the question of 'efficiency because if it's not 100% it must be 0%". Not that they say the last part out loud, of course.