The baby new to earth and sky, What time his tender palm is prest Against the circle of the breast, Has never thought that "this is I:"
But as he grows he gathers much, And learns the use of "I," and "me," And finds "I am not what I see, And other than the things I touch."
So rounds he to a separate mind From whence clear memory may begin, As thro' the frame that binds him in His isolation grows defined.
This use may lie in blood and breath, Which else were fruitless of their due, Had man to learn himself anew Beyond the second birth of Death.
A new baby has no concept that he is a separate, individual being from others. That's why he has no problem demanding (loudly and stink-ily) to have everyone wait on him hand and foot, right?
As the baby gets older, though, he starts to understand that he is an "I" and starts to separate himself from the "not me" things that he sees and touches.
At this point, the baby starts to develop a memory (presumably after the soft spots mentioned in the previous canto have closed up).
Tennyson points out in the last stanza that living would be worthless if memory is wiped out after death. You see, he's really, really, really hoping that Arthur hasn't forgotten about him up in Heaven.