Source identified1
“The future is no more uncertain than the present.”
Whitman refuses to treat the future as uniquely unknowable: within the poem's larger meditation on what endures, present conditions are themselves unstable and changing, so uncertainty belongs to both horizons.
Source note: Leaves of Grass (1882), “Song of the Broad-Axe,” section 4; Wikisource 1882 transcription. The line appears after “And the dead advance as much as the living advance” and before Whitman turns from institutions and structures toward the personal qualities he says endure.