David Allen opens by identifying unprocessed, unclear commitments held only in your mind — rather than captured in a reliable external system — as a significant source of mental clutter and stress that undermines genuine focus and productivity.

Why unprocessed mental commitments prove so costly:

• Holding unclear commitments only in memory requires ongoing background cognitive effort to avoid forgetting them, subtly undermining full attention to your current task
• This background mental load persists even when you're not actively working on these commitments, creating chronic low-level distraction and stress
• Allen's core insight is that capturing these commitments in a reliable external system frees this background cognitive capacity, providing genuine mental clarity

This opening establishes the book's foundational psychological insight, motivating its subsequent detailed practical system for externally capturing and processing commitments.