Tell me not, in mournful numbers,
Life is but an empty dream!
For the soul is dead that slumbers,
And things are not what they seem.
Life is real! Life is earnest!
And the grave is not its goal;
Longfellow expands upon the  life-as-dream metaphor in the first two lines  to say that if that comparison holds true, then our souls are already deceased. 
 
 “Slumber” was a favorite word of Longfellow’s; he used it to translate Dante’s Italian word  sonno  (“sleep”) in  his translation of the  Commedia .
A Psalm of Life