Obviously, this is about the perils of immortality. It's all very well living for ever, but what about those around you, loved ones, etc.? At the end of the song (the last nine lines), his wife is described as "withered", with "toothless" and "haggard" features. This is assuming such a man (could be a woman these days) remains faithful all that time. I think the line that really sums up the song is "a laugh as close as any tear". This is the title track to the album, and if you don't own it already, go and buy it now! Sheer poetry, brilliant music, one of the best singers ever, and the epitome of prog rock.