A useful seer
There's a tendency to think that the ancients must have been a gullible lot - all of that digging in entrails and watching the flight of birds to determine the future, but perhaps they were cleverer than they looked.
The Greek word usually translated as seer is mantis (which in ancient Greek also referred to the spider, which suggests the human practice required lots of sitting around, waiting and watching). Oracle is Latin.
The most famous mantis was Blackfoot (Melampous). Held in a prison cell, he called for the guards and got out seconds before the roof fell in - the story goes that the woodworms had "told" him they were chewing through the beam. Perhaps they used a trail of sawdust ...
He went on to cure a king's impotence, "having learned" from a vulture, that when he was a boy the king's father had threatened him with a gelding knife then left in the bark of a particular tree. He then prepared a solution of rust from the knife that restored the king's virility. No modern psychiatrist could come up with a better explanation and treatment.
This from a fascinating piece in the London Review of Books. You can read the full article, dense but worth sticking with, here. It is a review of The Road to Delphi: The Life and Afterlife of Oracles, by Michael Wood, Chatto, 2004.