The two ravens are Huginn and Muninn, the birds of the Norse God, Odin. He sends them out to gather information every day. They represent reasoning (conscious thought) and intuition (unconscious thought). There's a passage in the Grímnismál where Odin expresses his fears that when he send his ravens out they won't come back. Theory is that is a representation of shamanic practices and the fear that Odin's mind won't come back from the other side.