The soul of permaculture, its animating force and source of ethos and direction, is its intentional ethics of "earth care, people care, fair share" and its fundamental focus on decentralized, local sustainability. Truly, these beliefs elevate the whole exercise beyond 'just another way to plant tomatoes.'
However, I would say that the bones of permaculture - the structures by which it transmits the moral impetus of noble ideas, and transforms good intentions into magical results - the bones come to us in certain basic principles of action. One of the most powerful principles is stacking functions.
The principle of stacking functions dictates that, in a given system, each element should perform multiple duties. Understand from the beginning that this is profoundly different than
"multitasking", that mostly-mythic story we tell ourselves to justify
the insanity of trying to do eight separate tasks at one time,
effectively doing none of them well or with attention. Multitasking is
trying to do multiple tasks. Stacked functions is succeeding in being multiple things.
An easy mapping out of the concept comes from visualizing every element of a system in terms of the same basic characteristics - what does it need? what does it produce? what does it do? - and connecting those needs, products, and actions into other outputs, needs, and places in the system.
When I planted a dwarf pear tree in my front yard, I stacked many useful functions. First, the dwarf pear produces edible fruit, meeting my need for good food. Second, during the heat of the summer, its leafiness will give partial shade to the area around it, protecting it from overpunishment of late summer sun. So as it absorbs the energy it needs, it simultaneously creates a friendlier environment for less heat-tolerant plants below. Third, its root system will help hold the soil on the slope of the front yard, and will more effectively tap deep moisture than the shallow rooted plants around it, helping the soil stay moist and allowing better development of undisturbed soil structure. Fourth, its leaf-dropping habits will return useful biomass to me every year. Fifth, it serves as a relatively inexpensive teacher for me to learn more about cultivating fruit trees.
To multiply the utility of the dwarf pear, I put other plants in relationship with it. I planted some low-growing blueberry bushes around it. First, obviously, they produce delicious and healthy food, and a food that can be expensive to purchase. Second, they evolved to root shallowly in organic-rich soil on forest floors, so they are quite happy to live in the heavy mulch around the tree base (so the mulch is also stacking functions, doing double duty as a protective layer and as a rooting soil) and thrive in the sunny-but-not-vaporizingly-hot environment provided around the tree, as they like cooler soil. Third, both they and the pears make use of pollinating insects, so planting them close together increases the local density of honeybees and the like. Fourth, like the tree, as perennial plants, they will improve the soil in the long term by holding together a deeper, undisturbed soil structure. Fifth, as moisture-loving shrubs that don't like to drown in too much water, the tree helps keep them moist (tapping deeper water and bringing it to the surface via its roots and its transpiration) while at the same time providing a "sink" for excess water in the area. Finally, the blueberry bushes' shallow root spreads help suppress weed growth and grass from growing too near the tree base, which helps the pear tree, as certain grasses chemically retard tree growth.
To multiply the utility of both systems, let's put them in good relationship with me. I get exercise and satisfaction from planting and maintaining them, as well as super nutritious, homegrown, completely pesticide-free food - no food miles here, we're talking food-steps. Hugelkultur in both beds ensures greater health for plants, and a use for previously wasted scrap deadfall. I learn a great deal about the care and feeding of fruit crops, a skill I really want to develop. My landlord gets long-term value increases on his property, which encourages him to let me do more radical alterations to the yard. My neighborhood, a somewhat sketchy place at points, gets a small increase in beauty and stability. Wildlife get more places to nest and hide. Bees get food. A well-done system of interlocking, interdependent, multiply redunant (so the failure of any one element can't crash the works) stacked functioning systems not only builds a powerfully resilient permaculture system, but also creates momentum in a system - the better the design is, the less and less maintenance it requires to keep going.
So these examples mostly show the physical side of function stacking, but it runs deeper, much deeper, than just clever aligning of inputs and outputs. It produces a change in perception, creating not only a vision that sees connections rather than isolated elements, but a tolerance for the inevitable chaos present in a system. I'm digging a hugelkultur bed, and keep turning up big pieces of fieldstone. Since I think in terms of stacked functions, however, this isn't a source of frustration and cursing the ground; it is a source of free stones, which I'll use to line beds, build garden paths, and so forth. Now I can be thankful that the garden is providing for another need, that I hadn't even thought of, so I don't spend time and money and gasoline going to buy premade generic paving stones at a store somewhere.
Good examples of stacked functions abound, and I'll be trying to tie in this principle as I discuss planting choices, bed designs, and so forth. Some additional links with good examples:
Stacking functions with plant guilds:
An example almost humorous, and almost divine, in its simple straightforwardness:
One of the most accessible-to-beginners-but-still-useful-to-experts examples, the Three Sisters: