Reason, as used in this article, refers, collectively, to mental faculties that generate or affirm propositions, by activities of the mind such as judging, predicting, inferring, generalizing, and comparing.

Reason in this sense is often contrasted with authority, mysticism, superstition, and faith, and is thought by rationalists to be more reliable than these in discovering what is true or what is best. The meaning of the word "reason" overlaps to a large extent with "rationality" and the adjective of "reason" in philosophical contexts is normally "rational", rather than "reasoned" or "reasonable".

The precise way in which reason differs from emotion, faith, and tradition is controversial. Reasoning may be conscious or unconscious, it may be done mentally or with the steps written out. The concept 'reason' is closely related to the concepts of language and logic, as reflected in the multiple meanings of the Greek word "logos", the root of logic, which translated into Latin became "ratio" and then in French "raison", from which the English word "reason" was derived.

A reason is an explanatory or justificatory factor. In the context of explanation, the word "(a) reason" can be a synonym for "(a) cause".

Reason compared to logic, cause and effect thinking, and symbolic thinking

In modern times, there is an increasing tendency to use the terms "logic" and "reason" interchangeably in philosophical discussion, or to see logic as the most pure or the defining form of reason.

Reason and logic can be thought of as distinct, although logic is one important aspect of reason. Reason is a type of thought. Logic, as the word is used in modern languages, involves the attempt to describe rules by which reason operates, so that orderly reasoning can be taught. The oldest surviving writing to explicitly and at length consider the rules by which reason operates are the works of the Greek philosopher Aristotle, especially Prior Analysis and Posterior Analysis. Although the Ancient Greeks had no separate word for logic as distinct from language and reason, Aristotle's neologism "syllogism" (syllogismos) identified logic clearly for the first time as a distinct field of study. When Aristotle referred to "the logical" (hê logikê), he was referring more broadly to rational thought.

Author Douglas Hofstadter, in Gödel, Escher, Bach, characterizes the distinction in this way. Logic is done inside a system while reason is done outside the system by such methods as skipping steps, working backward, drawing diagrams, looking at examples, or seeing what happens if you change the rules of the system.

Another way to consider the confusion between logic and reason is that computers and animals sometimes perform actions which are apparently logical: from a complex set of data, conclusions are achieved which are "logical". Being a cause of something which humans find logical does not necessarily mean that computers or animals have reason, or even logic in the strict sense. Some animals are also clearly capable of a type of "associative thinking"—even to the extent of associating causes and effects. A dog once kicked, can learn how to recognize the warning signs and avoid being kicked in the future. Human reason is something much more specific, requiring not just the possibility of associating perceptions of smoke, for example, with memories of fire, but also the ability to create and manipulate a system of symbols, as well as indices and icons, according to Charles Sanders Peirce, the symbols having only a nominal, though habitual, connection to either smoke or fire.