"5D (Fifth Dimension)" was the most improbable single ever issued by the Byrds and, even more so, the most daring opening track ever on any Byrds album. More challenging than the single "Eight Miles High" which preceded it -- which had a driving beat and an extrovert quality -- "5D"'s lyrics were steeped in notions of trans-dimensional existence and planes of perception beyond length, width, depth, and conventional time. The song grew out of Roger McGuinn's reading of a deeply philosophical Don Landis book entitled 1-2-3-4, More More More More, which dealt with these questions of perceptions and reality. McGuinn turned these notions into a guitar-heavy and (for the first time in the band's history) organ-based tune (the latter instrument courtesy of Van Dyke Parks), its lyric expressing wonder and exultation. The group and Columbia Records had enough confidence in the record to try it as the first single culled from the album, though it never charted -- and they had enough confidence to lead the album off with it. Given the sheer eclecticism of what followed, it was an inspired choice, telling the listener that this was a record filled with strange states of mind and visions of the world, most of the beautiful.