我们之所以将自然界分解，组织成各种概念，并按其含义分类
抽象
All programming languages provide abstractions.
Five basic characteristics of Smalltalk
- Everything is an object.
- A program is a bunch of objects telling each other what to do by
sending messages. - Each object has its own memory made up of other objects.
- Every object has a type.
- All objects of a particular type can receive the same messages.
an even more succinct description of an object:
An object has state, behavior and identity.