~~
Who is the third who walks always beside you?
When I count, there are only you and I together
But when I look ahead up the white road
There is always another one walking beside you
Gliding wrapt in a brown mantle, hooded
I do not know whether a man or a woman
--But who is that on the other side of you?

from The Waste Land
by T.S. Eliot
Part V, What the Thunder Said, lines 359-365

In his notes Eliot alluded to the account in Luke 24, lines 13-16 of an unseen resurrected Christ walking with two of his disciples on the road to Emmaus. Allusion is also made to Shackleton's passage involving three of the Antarctic explorers imagining a fourth by their side. (less info)