Scientific Facts:
Mercury is the first planet from the sun and the smallest major planet of the solar system.  It revolves around the sun with a period (sidereal) of 88 days, has no known satellites and appears to rotate once every 59 Earth days.  Its surface, unprotected by an atmosphere is at a very high temperature when subjected to the rays of the sun and at a very low temperature elsewhere.
 

Rules:
The brow, logic, division, speech, union and separation, cleverness, crafts, ambiguity, adaptability, expression, merchandise, speed, communication, nerves, lungs, thieves, paranoia.
 

Quality:
This planet rules all communication, whether by speech, writing or lecturing,  It is the ruler of thought the ability to compare two things and therefore, by extension, many things.  At its core, Mercury is a dualistic planet.  This should develop into the power of thought.
 

Childhood Development:
Mercury rules the period when a child has just been born.  While aware of separate existence, he or she is still closely connected to the mother.  The arising of a sense of difference between child and Mother is the basis for the concept of division and the source of logical thought.
 

In People:
Mercurial people are clever, good communicators, skilled in crafts and language, enjoying ambiguity and tricks.  If Mercury is afflicted, the person may be unsure as to whether the world is real and she or he is real.  Given this mechanism, feelings may be projected onto the world.  This type of narcissism together with the overemphasis on calculation may lead to paranoia.