Good design takes variables into account. Whether you're designing a website, a building, a marketing campaign, a lesson, or a life, the things you know are just as important as the things you don't know. But even more important is to consider the things you think you know that it turns out you did not. Or, as Donald Rumsfeld puts it: The Unknown Knowns.
It's easy to say—when things are going well—that it was "all part of the design." But when things break, when accidents happen, when human being do dumb things, what will you say then? Responsible planners, creators, architects, designers, will still respond: "it was all part of the design."
Because stuff happens! Things outside of your control will happen. Cars will run into you even if you have your wits about you and are driving the speed limit. But that's why you have insurance, to plan for the unknown knowns, that is to say, the things you think know (good driving = zero accidents) that it turns out you did not (some high schooler ran the red light on his way to school and hit you).
Then there are known knowns. Things you think you know that you actually know. Like controlling how you react to the unknown knowns. How do you feel when accidents happen? Surprised? Disappointed? Frustrated? You may only be making things worse. You have the freedom, a radical capacity to choose your thoughts and beliefs at any given moment.
"You have power over your mind—not outside events. Realize this, and you will find strength." - Marcus Aurelius
This is the art of design.