Our world and that of the spirit realm cannot exist in the same space. Only on the night of Samhain does the veil part and allow souls to move freely from one realm to the other, but exceptions exist.
 
The Raven may travel from one world to another, carrying messages from the living to their ancestors, offering solace and words of love and longing.
 
Greet them respectfully when you meet as they may be on a special message for Hekate or for Morrigan, the Goddess of Death. 
 
Always be polite as you never know with whom you may be speaking!