The act of isolation involved [in concept-formation] is a process of
abstraction: i.e., a selective mental focus that takes out or separates a
certain aspect of reality from all others (e.g., isolates a certain attribute
from the entities possessing it, or a certain action from the entities
performing it, etc.).
The higher animals are able to perceive entities, motions, attributes, and
certain numbers of entities. But what an animal cannot perform is the process
of abstraction—of mentally separating attributes, motions or numbers from
entities. It has been said that an animal can perceive two oranges or two
potatoes, but cannot grasp the concept “two.”