>Documents work
 >> because you can use them early (models that build knowledge),
 >> because they persist (you're not crippled by your imperfect memory),
 >> because they're efficient (you don't have to keep repeating the same
 >> conversation with perfect fidelity), because they can capture
 >> details (not just vague impressions), because they can be reviewed,
 >> critiqued, and corrected (unlike your trembling thoughts), because
 >> they remain (unlike you, you job-hopper!), etc.

Excellent points. Extreme programming demands this of the code as well as any documents the customer may require.