Abstraction
The process of abstraction refers to the ability of conceptual beings to selectively focus upon different attributes of perceived entities, and to mentally group these attributes together, thus allowing the formation of concepts. On the perceptual level of awareness all that is provided is the entire object which has certain similarities and certain differences from other objects, which cannot be separated out without the power of abstraction.
Abstraction involves a certain kind of differentiation, that is distinct from that as exists on the perceptual level of awareness. As abstraction is fundamentally a conceptual process, it is not automatic---the "differentiation" on this level is not "this entity is distinct from that entity," but is rather a process of retaining the aspects whilst dropping the measurements.
Such differentiation cannot be performed arbitrarily. For example, one can form a concept by distinguishing tables from chairs, but not by distinguishing tables from red objects. There is no basis on which to bring these two sets of concretes together before the mind, and no way to identify a relationship between them. The reason is that the relationships required for concept-formation are established quantitatively, by means of (implicit) measurements---and there is no unit of measurement common to table-shaped objects and red objects. The attributes of shape and color are incommensurable.[1]
What is necessary to perform such an abstract differentiation (i.e. differentiation on the level of abstraction), is for the groups of existents to share a conceptual common denominator---the characteristics of each group must be reducible into the same unit of measurement.
The cognitive role of abstraction, therefore, cannot be easily under-stated. Abstraction allows man to group different concretes according to their similarities, no matter the differences between them---man can isolate these relevant aspects and thereby regard any given object not as a mere entity, but as a unit. An animal may be able to recognise such similarities and differences between entities, but "it cannot do anything cognitively with the relationships it perceives. To its consciousness, the noting of similarities is a dead end. Man can do something: he makes such data the basis of a method of cognitive organization. The first step of the method is the mental isolation of a group of similars."[2]