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Aclass is a collection whose members either fall under a predicate or are classified by a rule. Hence, while a set can beextensionally defined only by its elements, a class has also an intensional dimension that unites its members. When the term 'class' is applied so that it includes those sets whose elements are intended to be collected without a common predicate or rule, the distinction can be indicated by calling such sets "improper class."
Philosophers sometimes distinguish classes fromtypes andkinds. Theclass of human beings is discussed, as well as thetype (ornatural kind), human being, or humanity. While both are typically treated asabstract objects and not differentcategories of being, types not classes are usually treated asuniversals. Whether natural kinds ought to be considered universals is vexed; seenatural kind.
Types and kinds are discussed differently.Socrates is considered atoken of a type (or aninstance of the natural kind, human being) but amember of the class of human beings. He is a token (instance) not member of the type (kind), human beings. He is a member not type (or kind) of a class. The terminology is that types (or kinds) have tokens (or instances) while classes have members.
A class is conceptualized similarly to a set defined by its members.[1] The class is extensional. A set defined intensionally is a set of things that meet some requirement to be a member. Such a set creates a type. It also creates a class from the extension of the intensional set. A type always has a corresponding (potentially empty) class, but a class does not necessarily have a corresponding type.
