Mercury, Lord of COMMUNICATION, is symbolic of the faculty of discursive, analytical thought and thus rules over discrimination. Mercury is essentially our ability to discern Truth from untruth and to organize the events and aspects of life so that they make sense in light of our understanding of the Real. Thus Mercury plays a very important role in the religious and philosophical outlook (both a set of ideas/beliefs, aka mental constructs) of the native because it refers to the types of doctrines by which he/she cognizes the meaning of life.
In the Sufic arrangement it is regarded as the power of thought (al-fikr) which organizes and explains things according to its identification with matter — or — its identification with Spirit. The oscillation of thought between these two poles (spirit-matter) is symbolized by the long attributed dual nature of Mercury. Never far from the Sun, when Mercury is very near, it is symbolic of thoughts communing with The Intellect (Nous), while when it reaches its maximum distance it is like the mind contemplating the external (from the center of the 'circle of existence' to its circumference and back).
“The faculty which is specific to man is thought (al-fikr). Now the nature of thought, like the nature of man, is two-faced. By its power of synthesis it manifests the central position of man in the world and so also his direct analogy with the Spirit . . . In fact thought never plays an entirely “natural” part in the sense of being a passive equilibrium in harmony with the cosmic surroundings. To the degree that it turns away from the Intellect, which transcends the terrestrial plane, it can only have a destructive character, like that of a corrosive acid, which destroys the organic unity of beings and of things . . . This double property of thought corresponds to the principle which Sufis symbolize by the barzakh, the “isthmus” between two oceans. The barzakh is both a barrier and a point of junction between two degrees of reality. As an intermediate agent it reverses the pencil of rays of the light it transmits in the same manner as does a lens. In the structure of thought this inversion appears as abstraction. Thought is only capable of synthesis by stripping itself of the immediate aspect of things; the more nearly it approaches the universal [the Spirit (Sun)], the more it is reduced as it were to a point. Thought thus imitates on the level of form—and hence imperfectly—the essential “stripping bare” (tajrīd) of the Intellect”.
— Introduction to Sufi Doctrine, by Titus Burckhardt