The glyph of Mercury consists of the circle that represents Spirit, the semicircle that represents the Soul, and the cross that represents Matter.
Soul is above, Spirit is in the middle, and Matter is below in the glyph of Mercury.
This triplicity reminds us of Hermes Trismegistus (thrice-great Hermes), and Mercury is indeed thoughts, ideas, communication, and is the conduit and the herald between the layers of self and the planes of being.
The position of Mercury in a particular Sign and House shows us how the stream of consciousness is articulated into words, images, and ideas.
It is the way in which we synthesize, categorize, separate, and divide the world into hemes and things that can be named.
Mercury is never more than 28° from the Sun, sometimes in the same Sign as the Sun, and sometimes in the Sign before or after that of the Sun.
This is more than simply a matter of celestial mechanics; it is an indicator of the fact that waking consciousness can only be a certain distance away from the source of being which is the Sun.
Mercury through the power of thought expresses a blending of the capabilities inherent in the Sun and the Moon.
Just as Mercury shows us how we create the internal dialogue of consciousness, it also shows us how we communicate with others.
Mercury is associated with the sphere of Hod in the tree of life, which is the sphere of thought and all systems of notation and knowledge.
It is also connected to Hermes, Thoth, and other divinities that act as messengers and psychopomps.
Just as heralds have their badge office that allows them to safely move through places where others would be barred, and to communicate safely with powers that are potentially dangerous, so too in Magick do we find the Planet Mercury a useful intermediary in numerous operations.
Mercury is neutral and can be said to be all polarities and no polarities at once.