Here, Whitman defines truth by perspective...our own. Truth is. Typically, we define truth in terms of its consistency with the origin...it is what it appears to be. Whitman offers an alternative steeped in wisdom and insight. Truth is what we experience...see, feel, taste, smell, hear. The quality of these truths are revealed over time...they always return perfectly. So let us take his wise advice: absorb everything, deny nothing, and discern what experience teaches.
As I read Whitman here, I believe his gives us a poet's lament...what a mess we have become. A patriotic celebration, grand enough for the dead heroes of past to return. yet these heroes are not happy. Their protest is mighty. And so Whitman brushes them away. Nothing is to spoil this patriotic, Boston celebration. But then the poet gives away his real intent...the real rye twist of his interest. This celebration would only be complete by making ol' King George the centerpiece; To give the King his revenge. How? By observing what we have become. Could his tyranny have done worse?