Definition
Law of least effort is the principle that, all else equal, humans gravitate to whichever available action requires the least energy. It is not laziness — it is the basic economics of an organism that evolved to conserve calories. Modern environments are still being navigated by ancient hardware that prefers the path of least resistance.
The corollary for habit design is direct: arrange the environment so the desired behavior is the easiest path, and let the law work for you. Fighting the law with willpower is expensive and unreliable; redesigning the path is cheap and durable.
Why it matters
How it works
Effort is felt by the brain as a kind of subjective cost, summed across physical work, decision overhead, and ambiguity. The smaller this cost is at the moment a habit is supposed to fire, the more reliably it fires. Each extra step — finding the right tool, deciding what version of the thing to do, getting past a small obstacle — adds cost and gives competing behaviors a chance to win.
Habit designers thus front-load the work. The exercise clothes are laid out the night before. The book is on the nightstand, not in the office. The phone is in another room. None of these changes alter the persons motivation; they simply change which behavior costs less in the moment of choice. The law of least effort handles the rest.