The song develops one main theme from specific to general, or from question to answer, or from problem to solution.
Define the main theme in one sentence. Each verse adds a new layer: 1 = personal, 2 = broader context, 3 = universal truth.
Thematic arc gives the song meaning and direction. The listener feels the song is 'going somewhere.'
'Imagine': verse 1 = no heaven, verse 2 = no countries, bridge = no possessions.