Dale Carnegie opens with his central practical principle: living in 'day-tight compartments' — focusing attention and energy on the present day alone, rather than simultaneously carrying the burden of past regret and future anxiety.

Why this day-focused framing matters:

• Simultaneously carrying both past regret and future anxiety, alongside present demands, creates an unsustainable, overwhelming psychological burden
• Deliberately focusing attention on the present day's actual demands, setting aside past and future concerns during that focused time, provides genuine practical relief
• This principle, though simply stated, provides the foundational practice underlying much of the book's subsequent specific guidance

This opening establishes the book's foundational practical principle, motivating its subsequent detailed guidance for managing worry and anxiety.