The ability to regard entities as units is man’s distinctive method of cognition, which other living species are unable to follow.
A unit is an existent regarded as a separate member of a group of two or more similar members. (Two stones are two units; so are two square feet of ground, if regarded as distinct parts of a continuous stretch of ground.) Note that the concept “unit” involves an act of consciousness (a selective focus, a certain way of regarding things), but that it is not an arbitrary creation of consciousness: it is a method of identification or classification according to the attributes which a consciousness observes in reality. This method permits any number of classifications and cross-classifications: one may classify things according to their shape or color or weight or size or atomic structure; but the criterion of classification is not invented, it is perceived in reality. Thus the concept “unit” is a bridge between metaphysics and epistemology: units do not exist qua units, what exists are things, but units are things viewed by a consciousness in certain existing relationships.
With the grasp of the (implicit) concept “unit,” man reaches the conceptual level of cognition which consists of two interrelated fields: the conceptual and the mathematical. The process of concept-formation is, in large part, a mathematical process.
A “number” is a mental symbol that integrates units into a single larger unit (or subdivides a unit into fractions) with reference to the basic number of “one,” which is the basic mental symbol of “unit.” Thus “5” stands for |||||. (Metaphysically, the referents of “5” are any five existents of a specified kind; epistemologically, they are represented by a single symbol.)