What is Gestalt Psychology?

Arnheim (1986, p. 823) summarizes gestalt theory thusly:

Gestalt theory is concerned primarily with the complex dynamics of organization in field situations be they physical or psychological. This dynamics is not fully describe by the tendency toward simple, regular, symmetrical structure but requires acknowledgement of a countertendency that meets tension reduction with tension enhancement. The countertendency articulates physical and psychological units or objects in interaction with the equilibrating force...gestalt psychologists...were very much concerned from the beginning with biological, cognitive, and aesthetic reward of gestalt processes, namely the creation of well-functioning, stable, and clarifying patterns in nature, science, and art—a perfection difficult or impossible to obtain otherwise.