A class is a collection whose members either fall under a predicate or are classified by a rule. Hence, while a set can be extensionally defined only by its elements, a class has also an intensional dimension that unite its members. When the term 'class' is applied such that it includes those sets elements of which are intended to be collected without a common predicate or rule, the distinction can be indicated by calling such sets "improper class."
Philosopher A philosopher is a person who practices or investigates philosophy. The term ''philosopher'' comes from the grc, φιλόσοφος, , translit=philosophos, meaning 'lover of wisdom'. The coining of the term has been attributed to the Greek th ...
s sometimes distinguish classes from types and kinds. We can talk about the ''class'' of human beings, just as we can talk about the ''type'' (or ''natural kind''), human being, or humanity. How, then, might classes differ from types? One might well think they are not actually different categories of being, but typically, while both are treated as
abstract object In metaphysics, the distinction between abstract and concrete refers to a divide between two types of entities. Many philosophers hold that this difference has fundamental metaphysical significance. Examples of concrete objects include plants, ...
s, classes are not usually treated as
universals In metaphysics, a universal is what particular things have in common, namely characteristics or qualities. In other words, universals are repeatable or recurrent entities that can be instantiated or exemplified by many particular things. For ex ...
, whereas types usually are. Whether natural kinds ought to be considered universals is vexed; see natural kind. There is, in any case, a difference in how we talk about types or kinds. We say that Socrates is a '' token'' of a type, or an '' instance'' of the natural kind, ''human'' ''being''. But notice that we say instead that Socrates is a ''member'' of the class of human beings. We would not say that Socrates is a "member" of the type or kind, human beings. Nor would we say he is a type (or kind) of a class. He is a token (instance) of the type (kind). So the linguistic difference is: types (or kinds) have tokens (or instances); classes, on the other hand, have members. The concept of a class is similar to the concept of a set defined by its members. Here, the class is extensional. If, however, a set is defined intensionally, then it is a set of things that meet some requirement to be a member. Thus, such a set can be seen as creating a type. Note that it also creates a class from the extension of the intensional set. A type always has a corresponding class (though that class might have no members), but a class does not necessarily have a corresponding type.


External links

"Class" as analytical term in philosophy

* ttp://plato.stanford.edu/entries/logical-atomism/ "Class" as an aspect of logic, and particularly Bertrand Russell"s Principia Mathematica]
"From Aristotle to EA: a type theory for EA" quoted 26/10/2014.
Concepts in logic {{logic-stub