Whether we expand existing towns or build new cities it begs the question about how we build an environment that helps us live in harmony with each other and with the natural world, upon which we depend for our existence.The history of the earth told by the Bible has us beginning our journey in a garden and finding our destiny in a garden city.The last chapter has a vision of "the river of the water of life, bright as a crystal, flowing through the middle of the city". And on either side stands the Tree of Life with leaves for the healing of the people. It's a picture of harmony between the built and natural environments.Not so long ago I invited some of Liverpool's Leaders to meet an international expert on sustainable cities. Professor Herbert Giradet set out ten key principles on how to make cities liveable and sustainable.Up until that point I'd always seen cities in a linear way - gobbling up resources at one end and spitting them out at the other. The seminar helped me to see how waste could become energy. Cities could be circular, not linear, and mimic nature itself.