It deals with knowledge. Mercury is very knowledge-hungry. For example, after the first fire-burn a child staying away from fire is a mercurial learning. Because of his curious nature Mercury frequently plunges in unknown, and as a result, learns much more than others. Perhaps that s why detectives, spies usually have a dominating and well placed Mercury in their birth charts.
Mercury deals with all types of learning and acquiring of knowledge.
The basic first step of learning anything requires curiosity and interest, and Mercury meets that requirement. However, when it comes to learning at a deeper level, such as scientific research or invention, Mercury alone cannot handle it. Nevertheless Mercury is essential for any type of learning. The basic quality of observation, followed by logical interpretation of the observation, sharp analytical mind, general commonsense, and a keen sense of presence of mind are the attributes of Mercury. Mercury controls speech and power of speech.