NATURE. Reason is the ability of the mind to (1) integrate ideas--concepts, principles, and theories--from a multitude of discrete sense-perceptions of reality; (2) use various techniques for double-checking those ideas to make sure the mind has formed them logically; and (3) then apply those ideas to understanding and solving problems in life, whether they are narrow technical problems or universal ethical issues. Reason involves induction, deduction, integration, differentiation, questioning, analyzing, and other methods. This ability is reason in full.
Reason is, in summary, an ability (faculty) to engage in a process potentially involving many diverse steps, a process of integrating sense-perceptions and arriving at knowledge of the world and our proper role in it--that is, knowledge of facts and values.
EXAMPLE. A man was plagued by an escalating, thirty-year series of medical problems with his skin, eyes, joints, tendons, muscles, and intestine. His medical condition became worse. He became so crippled he could barely walk.
He stopped relying on the mostly ineffective advice of local physicians. As a layman, he began working on the problems himself. He thought about his symptoms and kept records of their ups and downs. He read books on each problem. He compared the symptoms he read about to what he could see in his own body. He asked himself questions about the possible cause of each problem. One day he discovered--that is, he integrated one piece of information with another--that the technical name of each problem had the same ending: dermatitis, iritis, tendonitis, and so forth. The "-itis" ending means inflammation. He wondered what might cause it.
One physician suggested that he eliminate a certain suspect category of foods. He did so, and he saw his problems diminish. He experimented with other foods. Within two years he was confident that proteins--in some form or level--triggered the problems. He cut back on proteins, and the problems greatly diminished. Then he made another connection: proteins are a kind of acid (amino acids). He wondered: Could acidity be a problem? He finally found a list showing the effect of certain foods on acidity in the body. He experimented again and found that the list worked as a predictor (though not an explainer) of inflammatory reactions. Accordingly, he eliminated all acid-producing foods by eating only fruit, vegetables, and potatoes. His medical problems gradually disappeared.
This layman used observing, integrating, differentiating, thinking, and questioning to solve his problems. He used reason. As a layman, and one with limited time to spare from his other interests in life, he did not reach expertise on his narrow subject, but he did make progress -- which is what reason provides. Reason is our tool for living on earth, which is the natural world, a world of things that each have identity.
FURTHER DISCUSSION. Ayn Rand's philosophy, Objectivism, is a philosophy of reason. All knowledge comes from reason and reason alone. The "Reason" entry in The Ayn Rand Lexicon neatly collects relevant excerpts of her writings about reason, beginning (as is usual with the Lexicon) with a definition and proceeding to elaborations, special problems, and applications. (The Ayn Rand Lexicon and all of Ayn Rand's writings are available through The Ayn Rand Bookstore.)
"For a definition of reason," Ayn Rand says, "see Introduction to Objectivist Epistemology." That work describes her solution to the central problem of epistemology, the origin of concepts (the "problem of universals"). Essentially reason is a conceptual faculty: forming concepts from percepts, and then building principles and theories, using logic as a guide. In the course of demonstrating her solution, she shows reason in action in its many forms -- induction, deduction, integration, differentiation, and so forth.
By contrast, any claims to knowledge coming from any source other than reason are instances of mysticism -- the subject of the next post.
Author, The Power and the Glory: The Key Ideas and Crusading Lives of Eight Debaters of Reason vs. Faith