The word 'reason' as used today is used ambiguous in its meaning. It may denote either of two mental faculties: a lower reason associated with discursive, linear thinking, and a higher reason associated with direct apprehension of first principles of mathematics and logic, and possibly also of moral and religious truths. These two faculties may be provisionally named Reason (higher reason) and rationality (lower reason). Common language and personal experience supply evidence of these being distinct faculties. So does classical philosophical literature, the locus classicus being Plato's Divided Line analogy.
The effect of currently using a single word to denote both faculties not only produces confusion, but has had the effect of decreasing personal and cultural awareness of the higher faculty, Reason. Loss of a sense of Reason has arguably contributed to various psychological, social, moral, and spiritual problems of the modern age. This issue was also a central concern of 19th century Transcendentalists, who reacted to the radical empiricism of Locke. It would be advantageous to adopt consistent terms that make explicit a distinction between higher and lower reason. One possibility is to re-introduce the Greek philosophical terms nous and dianoia for the higher and lower reason, respectively. This discussion has certain parallels with the recent theories of McGilchrist (2009) concerning the increasingly left-brain hemisphere orientation of human culture.