"Let nature take its course"
Renewable resources are those that are renewed and replaced by natural processes over reasonable periods, without the need for major non-renewable inputs. In the language of business, renewable resources should be seen as our sources of income, while non-renewable resources can be thought of as capital assets. Spending our capital assets for day-to-day living is unsustainable in anyone's language. Permaculture design should aim to make the best use of renewable natural resources to manage and maintain yields, even if some use of non-renewable resources is needed in establishing the system.
In restoring the balance between renewable and non-renewable resource use, it is often forgotten that these "new ideas" were the norm not so long ago. The joke about the environmentally aware person using a solar clothes dryer (washing line) is funny because it works on the very recent nature of much of this takeover of functions by technology and fossil fuels.
Renewable services (or passive functions) are those we gain from plants, animals and living soil and water without them being consumed. For example, when we use a tree for wood we are using a renewable resource, but when we use a tree for shade and shelter, we gain benefits from the living tree which are non-consuming and require no harvesting energy. This simple understanding is obvious and yet powerful in redesigning systems where many simple functions have become dependent on non-renewable and unsustainable resource use.
Permaculture design should make the best use of non-consuming natural services to minimise our consumptive demands on resources and emphasise the harmonious possibilities of interaction between humans and nature. There is no more important example in history of human prosperity derived from non-consuming use of nature's services than our domestication and use of the horse for transport, soil cultivation and general power for a myriad of uses. Intimate relationships to domestic animals such as the horse also provide an empathetic context for the extension of human ethical concerns to include nature.
The proverb "Let nature take its course" reminds us that human intervention and complication of processes can make things worse and that we should respect and value the wisdom in biological systems and processes.