The major principle of this book is that what we eat matters to us and our health, to our environment, and to our communities. Because it matters, we should be more aware of what we eat and should consider its impact.
Specifically, Pollan argues that we should be eating more locally produced food that comes from small farms. He believes that we should be eating from the "pastoral" food chain. The farms that make up this food chain use environmentally sensitive practices. They help our local economies. They also produce better quality food and do so in a more humane way.
The basic principle that underlies the book, then, is that what we eat matters very much and that we should, therefore, pay more attention to what we eat and where it comes from.