A fascinating song, IMHO. Firsly because it runs exactly on the edge between the '60s and the '70s, incorporating the best of both, then because it's one of the first Yes' songs going plainly beyond the traditional song structure: no verse, no chorus, no bridge... al is somewhat shuffled and still recognizable in a new and challenging order. The second half of "Then" is particularly exciting, with the instrumental part, a real war of the instruments set off by keyboards and drums, then expanding to the whole set and leading towards Jon Anderson's reprise of the main sung theme.
Yes in 1970.
This reprise is worth a note: Jon sings it with a whispered and broken voice creating one of the greatest vocal performances of his career. Of course, this is not yet symphonic rock, not exactly the way of prog Yes contributed to spread all around the world, but it's a creative and unpredictable music, full of soul and feeling. Something that left its mark. On music and on my own experience as a humble listener.