Discovering a unifying law of design in nature was not on my to-do list when I traveled to Nancy, France, in late September 1995. I was a forty-seven-year-old professor of mechanical engi- neering at Duke University who had come to deliver a lecture at an international conference on thermodynamics. Giving you a sense of how steeped my career was in mechanical engineering, I remember that I had brought flyers announcing the publication of my seventh book, Entropy Generation Minimization . My work took a fateful turn during the prebanquet speech delivered by the Belgian Nobel laureate Ilya Prigogine. Echo- ing the scientific community''s conventional wisdom, this famous man asserted that the tree-shaped structures that abound in nature--including river basins and deltas, the air passages in our lungs, and lightning bolts--were aléatoires (the result of throw- ing the dice). That is, there is nothing underlying their similar design. It''s just a cosmic coincidence. When he made that statement, something clicked, the penny dropped.

I knew that Prigogine, and everyone else, was wrong. They weren''t blind; the similarities among these treelike struc- tures are clear to the naked eye. What they couldn''t see was the scientific principle that governs the design of these diverse phe- nomena. In a flash, I realized that the world was not formed by random accidents, chance, and fate but that behind the dizzying diversity is a seamless stream of predictable patterns. As these thoughts began to flow, I started down a long, uncharted, and wondrously exciting path that would allow me to see the world in a new, and better, light. In the sixteen years since, I have shown how a single law of physics shapes the design of all around us. This insight would lead me to challenge many articles of faith held by my scientific colleagues, including the bedrock beliefs that biological creatures like you and me are governed by different principles from the inanimate world of winds and rivers and the engineered world of airplanes, ships, and automobiles. Over time, I would develop a new understand- ing of evolutionary phenomena and the oneness of nature that would reveal how design emerges without an intelligent designer.

I would also offer a new theory for the history of Earth and what it means to be alive. In addition, I and a growing number of scientists around the world would begin finding new ways to make life easier: better ways to design roads and transport systems; to predict the evolution of civilization and science, of cities, universities, sports, and the global use of energy. We would unravel the mysteries of Egypt''s Pyramids and the genius of the Eiffel Tower while demonstrating how governments are designed like river basins and how busi- nesses are as interdependent as the trees on the forest floor. All that lay in the future when I boarded the plane for the trip home. High over the Atlantic, I opened my notebook (the old- fashioned kind, with paper) and wrote down the constructal law: For a finite-size flow system to persist in time (to live), its configuration must evolve in such a way that provides easier access to the currents that flow through it. I was writing in the language of science, but the fundamental idea is this: Everything that moves, whether animate or inani- mate, is a flow system. All flow systems generate shape and struc- ture in time in order to facilitate this movement across a landscape filled with resistance (for example, friction). The designs we see in nature are not the result of chance.

They arise naturally, spon- taneously, because they enhance access to flow in time. Flow systems have two basic features (properties). There is the current that is flowing (for example, fluid, heat, mass, or infor- mation) and the design through which it flows. A lightning bolt, for example, is a flow system for discharging electricity from a cloud. In a flash it creates a brilliant branched structure because this is a very efficient way to move a current (electricity) from a volume (the cloud) to a point (the church steeple or another cloud). A river basin''s evolution produces a similar architecture because it, too, is moving a current (water) from an area (the plain) to a point (the river mouth). We also find a treelike struc- ture in the air passages of lungs (a flow system for oxygen), in the capillaries (a flow system for blood), and the dendrites of neurons in our brains (a flow system for electrical signals and images). This treelike pattern emerges throughout nature because it is an effective design for facilitating point-to-area and area-to-point flows.

Indeed, wherever you find such flows, you find a treelike structure. Since human beings are part of nature and governed by its laws, the point-to-area and area-to-point flows we construct also tend to have treelike structures. These include the transportation routes we follow to work (a flow system for moving people and goods), which include many smaller driveways and neighborhood paths flowing into a few larger roads and highways. So, too, do the flowing networks of information, material, employees, and customers that keep those businesses afloat. The engineered world we have built so that we can move more easily does not copy any part of the natural design; it is a manifestation of it. That said, once we know the principle, we can use it to improve our designs. Although treelike structures are a very common design in nature, they are only one manifestation of the constructal law. In a simple example, logs floating on a lake or icebergs at sea ori- ent themselves perpendicular to the wind in order to facilitate the transfer of motion from the moving air body to the water body.

A more complex example is the design of animals that have evolved to move mass better and better (to cover more distance per unit of useful energy) across the landscape. This includes the seemingly "characteristic" sizes of organs, the shape of bones, the rhythm of breathing lungs and beating hearts, of undulating tails, running legs, and flapping wings. All these designs have arisen--and work together--to allow animals, like raindrops in a river basin, to move more easily across a landscape. The constructal law dictates that flow systems should evolve over time, acquiring better and better configurations to provide more access for the currents that flow through them. Design gen- eration and evolution are macroscopic physics phenomena that arise naturally to provide better and better flow access to the currents that run through them. The majesty of this principle is that it occurs at every scale. Each component of an evolving flow system--each rivulet, each tree, each road--acquires evolving designs to facilitate flow access. As these elements coalesce into larger and larger structures (into evolving river basins, forests, and transport networks), the various-sized components work together so that everything flows more easily.

We see this, for example, in the shape and structure of the neural networks in the brain, of the alveoli in the lung, and the human settlements on a map. In the big picture, all the mating and morphing flows on the largest system that surrounds us, the Earth itself, evolve to enhance global flow. E pluribus unum (one out of many) . The constructal law is revolutionary because it is a law of physics--and not just of biology, hydrology, geology, geophys- ics, or engineering. It governs any system, any time, anywhere, encompassing inanimate (rivers and lightning bolts), animate (trees, animals), and engineered (technology) phenomena, as well as the evolving flows of social constructs such as knowledge, language, and culture. All designs arise and evolve according to the same law. This law tears down the walls that have separated the disci- plines of science by providing a new understanding of what it means to be alive. Life is movement and the constant morphing of the design of this movement.

To be alive is to keep on flow- ing and morphing. When a system stops flowing and morphing, it is dead. Thus, river basins configure and reconfigure them- selves to persist in time. When they stop flowing and morphing they become dry riverbeds, that is, the fossilized remains of earlier "live" flow systems. The solid, treelike veins of ore found under- ground today, for example, are fossils of the fluid streams, eddies, and meanders that flowed before solidification a long time ago. Biological creatures are alive until all their flows (blood, oxygen, locomotion, and so on) stop, after which they, too, become fos- silized remains. This unifying definition marks an advance because it removes the concept of life from the specialized domain of biology. It aligns it (or, better, it juxtaposes it) with the physics concept of the dead state, which means "equilibrium with the environ- ment" in thermodynamics: a system that is at the same pres- sure, the same temperature, and so forth as its surroundings, and hence, in which nothing moves.

The constructal law defines life in physics terms, and it covers all live-system phenomena. It also reframes the view that life on Earth began with the rise of primitive species some 3.5 billion years ago. As we will see, "life" began much earlier, when the first inanimate systems, such as currents o.