Oh the Prophet, Nomad, Hero, Artirst stuff is supposedly based on the cyclic nature of human development.

To take the Wikipedia entry for it:
A Prophet (or Idealist) generation is born during a High, spends its rising adult years during an Awakening, spends midlife during an Unraveling, and spends old age in a Crisis. Prophetic leaders have been cerebral and principled, summoners of human sacrifice, wagers of righteous wars. Early in life, few saw combat in uniform; late in life, most come to be revered as much for their words as for their deeds.
A Nomad (or Reactive) generation is born during an Awakening, spends its rising adult years during an Unraveling, spends midlife during a Crisis, and spends old age in a new High. Nomadic leaders have been cunning, hard-to-fool realists, taciturn warriors who prefer to meet problems and adversaries one-on-one.
A Hero (or Civic) generation is born during an Unraveling, spends its rising adult years during a Crisis, spends midlife during a High, and spends old age in an Awakening. Heroic leaders have been vigorous and rational institution-builders, busy and competent in old age. All of them entering midlife were aggressive advocates of technological progress, economic prosperity, social harmony, and public optimism.
An Artist (or Adaptive) generation is born during a Crisis, spends its rising adult years in a new High, spends midlife in an Awakening, and spends old age in an Unraveling. Artistic leaders have been advocates of fairness and the politics of inclusion, irrepressible in the wake of failure.