In the opening stanza, the speaker directly addresses the psalmist. He begins by dismissing the psalmist's sad poetry, and he rejects as dangerous the psalmist's notion that human life is a meaningless illusion. If one accepts the logic that life is just a dream, he cautions, one's soul will not merely sleep, but die. On the surface, human life may appear futile, but the speaker contends that it is actually this sense of hopelessness - and not human life itself - that is the illusion.
Lines 5-8
Longfellow uses the second stanza to build on the ideas of the first. Because the soul lives eternally, the speaker reasons, life must be real. Note that in the first line there is a caesura, or break, after the word "real." This caesura forces the reader to pause, thereby emphasizing the idea that life is real. These lines are an allusion to the Bible's book of Genesis, where God says to the fallen Adam, "dust thou art, and unto dust shalt thou return." In Longfellow's poem, the speaker is asserting that although the mortal body will die, the soul is exempt from death.
Lines 9-12
The third stanza introduces the central theme of the poem: the purpose of life is not to experience pleasure or sorrow, but "to act" - to perform the deeds that will improve the condition of mankind. Note that by this point in the poem, the speaker has ceased to address the psalmist; instead, he is directing his remarks to mankind in general, as is evidenced by his broadly inclusive use of the first person plural - "our" and "us."
Lines 13-16
The fourth stanza begins with an allusion to a line from Seneca's work De Brevitate vitae, which states "vita brevis est, ars longa," or "Life is brief, art long." The idea here is that although a lifetime passes relatively quickly, it actually takes a long time to learn how to live well - to decipher the "art" of living. The speaker is
