The Computation

For my first twenty years, since yesterday,
  I scarce believed thou couldst be gone away;
For forty more I fed on favours past,
  And forty on hopes, that thou wouldst they might last;
Tears drown’d one hundred, and sighs blew out two;
  A thousand, I did neither think, nor do,
Or not divide, all being one thought of you;
  Or in a thousand more, forgot that too.
Yet call not this long life; but think that I
Am, by being dead, immortal; can ghosts die?