Central to my area of research are many things that are called complex. Complexity science and complex networks for example. For this reason I have become quite aware of the meaning of the word and therefor of its misuses. So I thought I try to explain what complex means in relation to simple and complicated and why it is so important to understand the difference in the modern world - I am really tempted to write complex modern world but it is kind of unfair to write about explaining a word and then use it without having explained it.
So when are things simple? Sure enough one can use simple do describe many things. However there is a more or less good definition of what simple is in the context of complexity. Simple relationships between two things are those that one can easily understand, the ones that are obvious. One might say linear speaking in a more mathematical context. However I believe it is better to stay with the more vague definition of easy to understand and obvious. These might change from person to person but that only makes it clear that there is no clean border line between simple and complicated.
Complicated is in many ways the opposite of simple. This may sound impossible, since where is the place then for complex - but hear me out. Complicated things are those that we can not understand easily, that take time to think through. Some of the complicated problems are so hard that it might take years to understand them, but it is possible. Complicated problems are hard because they usually don‘t conform to our what we now about the world so far or because the chain of how one thing becomes another thing is much longer then we are used to.
Now that we have both of these words covered we can start to make some examples. Lets have a look at a car. Using a car seems fairly simple, at least in concept. Doing it in the real world takes some time to get used to but everyone can do it. Back to the concept however. In the simplest version you have two pedals and a steering wheel. You push one pedal to accelerate and one to decelerate, while you use the steering wheel to point the car in the right direction. Simple. Even if you have a manual and have to operate a clutch and a stick it is still easy enough to understand. If you look however how the acceleration part really works you will soon find out that it is not so simple. In modern cars it starts with a sensor that measure how far you pressed the pedal. This signal then gets processed by the ECU. Depending on the state of the car, it‘s speed, acceleration, steering angle, possible faults and diverse settings, the state of the environment like the temperature, oxygen in the air and legal requirements (hello VW scandal) the ECU then decides what to do. It does that by feeding all the data it gathers in to a software model of the physical engine it controls. This software model then computes how much fuel and which time to inject in the engine. And so on. Depending in the model of car you can go down this rabbit hole at least a few more miles deep until you hit rock bottom. At several points there will be things that are not happening the way one would expect them to be assuming things are generally simple. However given enough time, we can understand all parts of this process and they will enable us to understand how the car accelerates if we press the gas pedal down.