What is simplicity, exactly?

If today a car lacked a steering wheel, that’d be a problem. You would say the steering wheel is “essential” to the car. Omitting it would be an “oversimplification” - taking you where you want to go is a fundamental thing a car does, and steering is a critical part of that.

This implies that there is a certain degree of complexity that is essential.

On the other hand, this essential complexity can easily be needlessly exceeded.

The bells and whistles of the old fairground organs are the epitome of this. Bells and whistles are unnecessary, by definition. (It is worth nothing, though, that a bell or whistle can serve as a source of delight.)

Da Vinci said, “Simplicity is the ultimate sophistication.” What did he mean by this? I think part of what he meant is that there’s an opportunity to take the essential complexity of a given thing and hide some of it. Doing so involves hiding precisely the complexity that can be hidden, and no more. The goal is to expose the appropriate complexity - the bare minimum complexity that people must know about - and to hide away the rest.

This is not an easy task, which is why it requires sophistication. Such sophistication, if achieved, can make the process of simplification look easier than it really is.

One of the challenges is that the bare minimum amount of complexity in itself changes over time. There was a time when most Americans needed to know about the gears of the car - when everyone drove manual cars. Now, with most Americans driving automatics, most of that complexity can be hidden. In the future, it might make sense to fundamentally alter the steering wheel, for a similar reason.

To deeply understand all of the facets of something and then to strip away what is necessary implies that you have full understanding of the domain, understanding of the target audience, and then, after acquiring this knowledge, are able to apply the self-discipline to resist the temptation to expose your expertise (which seems helpful to you, but won’t be helpful to others). All of these things must be held in a delicate, sophisticated balance.