https://maximummathematics.com/books/foundational-mathematics/set-theory/subsets/index.htmlA set can contain a wide variety of objects. They can contain objects that most people interact with on a daily basis — automotive parts that can be used to service a 1967 Camaro, tools used to carve statues out of wood, or art supplies needed to paint a picture. Likewise, they can contain a wide variety of mathematical objects, like numbers, shapes, or axioms. Sometimes, we only care about some of the objects in a set. For example, we may only be interested in automotive parts needed to service a car’s headlights, or we may only be interested in art supplies needed for a fresco painting. Regardless, we’re able to build a lot of structure by constructing new sets from old ones — simply by restricting what elements are included in the new set. Here, we explore this kind of relationship between sets.Hugoen-US