The magical designer
Let's ask two questions here. One: What is design? Two: What is an explanation, such that design explains something?
To the first: when some person designs an object, they typically have several features as designers. They have a plan or intention, they have the competence to make that plan work, and they have the tools to implement the plan.
Plans don't generally pop up out of thin air. The most inspired of people have usually, if not always, spent years learning and pondering, trying out variations, and so on. As Pasteur said, chance favours the prepared mind. Artists train for years in older styles and techniques before they can make the break and move to novel ones of their own devising. Even what looks like random splatter, as in Jackson Pollock's Blue Poles, is the end result of years of experimentation. Plans get developed by inheritance of prior technique and goals, and by trial and error.
So, too, the competence. Anyone who trained in any art or skill knows that you don't do it easily at first, or well, but that you need to practice, identify your mistakes (or more likely have them identified for you by one who has already achieved proficiency), and correct them. Competence is the outcome of trial and error.
Tools and techniques in the manufacture (the word once meant "the doing of the hands") of designed objects are themselves the end result of a long period of cultural and technological development. One cannot make, say, a Toledo blade sword, without the knowledge of what materials to use, how to assemble and treat them, and the tools to cast and sharpen them. This is itself the end result of a process of, you guessed, trial and error, at the level fo the tradition, guild or community.
So ordinary design, the design we do know something about, and which features in our explanations of artifacts, is the end result of trial and error processes, which the American psychologist Donald Campbell called "variation and selective retention". He did this in direct parallel to a process of natural selection, and he added one more adjective - "blind". The variation cannot, ex hypothesi, be directed, because we do not know ahead of time what will work. So, we work blind, applying what we used before and making, deliberately or not, small changes to our techniques which may, or may not, work out. We make these variations in many ways. We try them out in our mind's eye, and reject those that seem wrong to us (based on the past). Sometimes this is not useful. We may discard approaches that could have worked very well indeed. But we will never know this, and the "space" of all possible variations is so vast that we just cannot try them all out.
Then we try the seemingly viable variations out in practice. Often we will start, and stop when it seems to be going wrong. This may also mean we miss out on good tricks. Again, the space of all variations is vast. We could also waste our lives trying everything. Then, if they work in a small scale, we will attempt to see if they work well in the wild, so to speak. If people find them useful swords or pleasing paintings. Then, we have advanced our tradition beyond where it once was. Thus is progress made.
Could it be that genius can sidestep this trial and error? I doubt it. Even the greatest of geniuses have, in their fields, studied. They may have a more creative mind, or be especially well-adapted to making bold moves that turn out to work, but before the event, they are indistinguishable from the many creative and bold folk who will eventually fail.
So I am going to make a declaration here; a principle of design, if you will. Call it Wilkins' Design Desideratum: any successful design is the outcome of past experience and trials.
Next, we'll consider how design is used in explanation.