Reason is sometimes narrowly defined as the faculty or process of drawing logical inferences. From Aristotle onwards, such reasoning has been classified as either deductive reasoning, meaning "from the general to the particular", or inductive reasoning, meaning "from the particular to the general". In the 19th century, Charles Peirce, an American philosopher, added a third classification, abductive reasoning, by which he meant "from the best available information to the best explanation", which has become an important component of the scientific method. In modern usage, "inductive reasoning" sometimes includes almost all non-deductive reasoning, including what Peirce would call "abductive". (See also logic, term logic.)
Reason has also been conceived more broadly. George Lakoff and Mark Johnson explicate reason and its scope in this manner:
Rationalists see reason as the faculty by which fundamental truths are intuitively apprehended. These fundamental truths are the causes or "reasons" that things exist or happen. Empiricists, of course, deny the existence of such a faculty.
For Immanuel Kant, reason (Vernunft in Kant's German language) is the power of synthesizing into unity, by means of comprehensive principles, the concepts provided by the intellect (Verstand). The reason which gives a priori principles Kant calls "Pure Reason" (as in his The Critique of Pure Reason), as distinguished from the "Practical Reason" which is specially concerned with the performance of particular actions.
In theology, reason, as distinguished from faith, is the human intelligence exercised upon religious truth whether by way of discovery or by way of explanation. The limits within which reason may be used have been laid down differently in different churches and periods of thought: on the whole, modern Christianity, especially in the Protestant churches, tends to allow to reason a wide field, reserving, however, as the sphere of faith the ultimate (supernatural) truths of theology.
Regardless of how it is conceived, reason has often been seen as a uniquely human trait, which separates us from the other animals.
These days, the idea of reason as an independent faculty of the mind, separate from emotions, and unique to humanity, is under attack from a number of sources. Consider, for example, George Lakoff and Mark Johnson's theories about the "embodied mind". (See the Lakoff article for more information.)