Lei (类) is used to refer to people or things which have similar attributes or characteristics. It represents recognition of the commonalities of things and the differences between categories. Establishing which categories individual entities belong to, and what distinctions and boundaries there are between different types is the basis for naming and developing an order of all things in the world. Many ancient schools of thought studied the concept of lei, and offered different views on the use of category names, the principles of categorization, and the relationships between objects in the same category.