Reason and Understanding are discussed in depth by Ralph Waldo Emerson in his work, Nature. Emerson believed that the ultimate end of Nature is the discipline of understanding and reason. Because of this idea, he believed that Nature did not have to exist independently of the mind--this was a part of his Idealism.
Emerson felt that Nature educates the Understanding and Reason. Every property of matter is a school for the understanding. Nature educates understanding by the analysis of these properties. Reason transfers lessons of Nature into its own world of thought. It percieves the analogy that brings together the properties of matter and the mind. Understanding is scientific, it tries to answer questions. It is man's intellect based upon what he sees in the relative world. Reason is conscience, or universal morality. It is based upon consciousness and transcends understanding.
Emerson said we must have both reason and understanding in our lives. Understanding is necessary in order to live in the world--with reason only we would be absolute idealists. Reason gives us access to innate ideas. It is the highest faculty of the soul--without reason's insights there can be no human progress. In a lecture he gave at the Masonic Temple in 1841, entitled The Conservative, he spoke of how war and the division of people through political parties and wealth levels were the opposition of Understanding and Reason.
Emerson broadened ideas of Kant. Emerson wrote that Kant had showed that there was an important category of ideas that were not brought on by experience but "through which experience was acquired." Kant said "All our knowledge begins with the senses, proceeds then to understanding, and ends with reason." According to Kant, Reason is the best, if not ony way to come unto knowledge. The senses and understanding may beging the process, but we need Reason to make judgements on what we sense and try to understand.
Plato's ideas on reason and understanding are similar to the idea that understanding tries to answer questions, while reason is conscience. Plato felt that understanding was parallel with science and scientists, they tried to understand truths about the world in empirical ways. Philosophers are joined by reason. They go beyond empirical data to contemplation--the aesthetic. Plato felt that reason reveals the order of the changing state of the appearance of the world, as well as creating "a harmonious and happy life." He believed that man can know the real world beyond "the changing shadows of real things" only by his reason.
Finally, we can look at a passage from Milton's Paradise Lost, which has been cited by Coleridge, to see the distinction between reason and understanding. Rapheal is explaining to Adam and Eve the difference between heavenly perception (reason) and earthly perception (understanding):
To vital spirits aspire, to animal,Lotus Wuensch
To intellectual, give both life and sense
Fancy and understanding, whence the Soul
Reason receives, and reason is her being....
See also:
Plato
Kant's Critique of Pure Reason
Kant's Critique
American Transcendentalism