The human Reason is a power of the mentality that attempts to find answers, to elicit truth, from the experience, facts and impressions of the senses, subject to the analytical and cumulating power of the Reason. This process necessarily involves serious limitations both as to the facts or impressions upon which the Reason is asked to work, and as to the inherent deficiencies of the human reason, in and of itself, based on the fragmentation and separation that is the foundation of the human mentality. The resultant inaccuracy and error is inherent in the process.
The supramental Reason, however, starts from a different standpoint and thus can yield vastly superior, accurate results. Sri Aurobindo observes: “The supramental reason is of the nature of a spiritual, direct, self-luminous, self-acting will and intelligence, not mental, …, but supramental…. It acts by the same four powers as the intuitive mind, but these powers are here active in an initial fullness of body not modified by the mental stuff of the intelligence, not concerned mainly with an illumining of the mind, but at work in their own proper manner and for their own native purpose. And of these four the discrimination here is hardly recognisable as a separate power, but is constantly inherent in the three others and is their own determination of the scope and relations of their knowledge. There are three elevations in this reason, one in which the action of what we may call a supramental intuition gives the form and the predominant character, one in which a rapid supramental inspiration and one in which a large supramental revelation leads and imparts the general character, and each of these raises us to a more concentrated substance and a higher light, sufficiency and scope of the truth will and the truth knowledge.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 21, The Gradations of the Supermind, pg. 790