All science is based on modeling decisions. It's deciding whether a particular model is suitable for purpose. If you're off by 1% of 1 degree, it's going to matter by the time you get there. So in object oriented programming, that begs to be an object is a state object that contains position and velocity. Now, under the hood, it is a pandas series that is mapping from names to values. Similarly, with a system object, that's actually a Python namespace. And it represents everything that you need to know about a system in order to model it. In addition to just having the books, you've also got a bunch of Jupiter notebooks that people can check out