Energy underlies every decision and enables all progress. From biological metabolism to economic growth to organizational dynamics, energy flow determines what's possible. The entire arc of human civilization is a story of accessing and deploying energy more efficiently. Understanding energy constraints means understanding the deepest limits on what systems can accomplish.

Living systems obey strict scaling laws determined by metabolic energy requirements. Kleiber's Law shows that metabolic rate scales to the 3/4 power of body mass—a mouse's metabolism per unit mass is far higher than an elephant's. These constraints explain why animals have size limits, why cities scale differently than organisms, and why energy efficiency changes with scale.