In math model, we often use formulas, models, and variables to represent various complicated stuffs in reality. For example, we can use a function to describe a population growth or use a matrix to represent a transportation network. However, we all know that all these models are used for abstract expression in reality. At here, the article mentions that 2 + 2 = 4 only works when all four units are identical, but, in reality, each unit has its own difference just like apples which have different sizes. So, we need to always have a crtical thinking and humble when we use these models and data. After reading this chapter, I have a question, in our daily lives, can we fully rely on data to make decisions or we have to leave some space for non-data decision since if all the data try to simplified reality