The placement of Mercury in Capricorn indicates you have a need to function mentally within set structures and to achieve goals. Much of the behavior and many of the personality characteristics associated with Mercury in Capricorn are a result of your need for mental organization and a focus on the concrete and the realistic. Mercury is generally well placed in Capricorn. This is symbolized by the natural trine between Capricorn and Mercury-ruled Virgo. Capricorn is the Cardinal Earth sign. Capricorn’s active earth quality resonates with the concrete and objective energy inherent in Mercury and symbolically forces Mercury’s changeable airy energy to apply itself to practical matters and to run along organized grooves. 

Inherent in the placement of Mercury in Capricorn, however, is a tendency for spontaneity and freedom of thought to be sacrificed. The earth of Capricorn symbolically traps and confines Mercury’s airy energy. If you are not careful to keep the Mercury-associated functions stimulated, these can easily “get into a rut” and become rigid and stale. 

Insecurities

With Mercury in Capricorn, if you are insecure, then your mind is likely to be unsure of its own power of independent thinking. You will not really trust your mind or yourself to set proper goals for yourself. Therefore, you will rely on established conventions and these will govern your mind. In actuality, your ego has subconsciously set up this defense mechanism to keep the mind away from the Truth, which threatens the ego.  This insecurity about the mind, however, can also result in feelings of inferiority, mentally or with respect to your powers of communication. Your feelings of inferiority are likely to be exacerbated by the natural Capricornian drive to for success, which you do not think is within your reach. 

Mercury in the 1st House:

Having Mercury in the 1st House suggests that your mind and your ability to communicate play an important role in defining your identity and your sense of self. Inwardly, you may mentally focus on your self-identity. You are apt to think consciously and analytically about your identity and selfhood. You are more likely to consciously form the question, "Who am I?" than are most people. You can be expected to take a rational approach to answering this question. With Mercury in the 1st House, your self-analysis may extend beyond this basic question to others involving the definition of your identity. The danger in your mental energy being particularly focused on your self-identity is that you may become too self-absorbed. 

With Mercury in the 1st House, tt is even more common for you mental and communication functions to manifest as an integral part of your identity. You are likely to see yourself as a thinking person (or, if you shun seeing yourself as an "intellectual," as a communicating person). Because the mental and communicating functions are so closely identified with your self-concept, they are likely to be energized and extroverted.