Going through David Holmgren's version of the Permaculture Design Principles, here's #3.
Obtain a Yield
We invest (our time, attention, energy) into systems that reward us, so it is important to make sure that the permaculture systems we design properly thank us. A food forest design that won't produce any food for the first 10 years needs to be rethought and integrated with an understory that fruits early and often, as well as a fair mix of annuals that will motivate the owner to get out and care for the maturing design. Yields provide the positive feedback that maintain the system.
And there are lots of yields:
Mollison started us out on this one - replace the non-edible landscaping with edibles! If you are going to have a tree, why not one that pays a dividend of food every year? Permies love the perennials because they take less work, but if you love your tomatoes then plant them too!
Show love for the critters big and small by welcoming them in. If we have bee-attracting plants in flower throughout the season, then the little pollinators are a lot more likely to be around when we need them. That's a yield, because hand-pollination kinda sucks. Forage for the deer can keep a handy protein source nearby, and it is one that finds its own food.
- System improvement
Build the soil, build the soil, build the soil. Oh yeah, also catch the water, catch the water, catch the water.
Love of it may be the root of all kinds of evil (seriously, get the quote right folks), but it is also how our civilization currently keeps score. I don't have any interest in withdrawing from the global economy, so I either need a big enough pile to buy whatever I need (unlikely), or my systems need to be designed with some kind of marketable output.
Include a yield for the soul - tell yourself that we don't know all of the useful connections between species, and throw in a few ornamentals just because.
Pretty picture of the principle at permacultureprinciples.com