"You can't work on an empty stomach"
This principle is like an order. Make sure that every time you design a house, garden, park, or school, it includes elements that will provide real tangible yields. The yields could be food, fibres, or fuels, but there needs to be something. It is crazy to live in settlements where the only food source is the shop, and to get to the shop you pass gardens and parks, filled up with ornamental plants and gravel.
Permaculture stresses self-reliance - the ability to meet many of our own needs from our own resources. In a high rise flat that might be a window box with lettuces, as a whole community it may be the majority of our food. We can no longer rely on global food systems to meet our needs, or on there always being enough fossil fuels to bring the crops to us.
Permaculture stresses the use of plants that are functional - food medicine, fibres, but this doesn't mean boring or dull. Functional designs and plantings can also be beautiful - another important yield.
As Bill Mollison says: "The yield of a system is theoretically unlimited, or, limited only by the information and imagination of the designer." Put another way, 'obtain a yield' is not something we do once, but rather an approach to how we run our homes and design our settlements. Its a creative process and means that we are always looking to see how we can make an improvement here, add another species there.
With high yielding homes and settlements, we can meet our needs with less land. Then we can give some back to the many other species that are currently being 'squeezed out' by our current energy and land intensive agricultural systems.