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.