Mercury is the planet associated with communication, information, education, disputation, logic and calculation. The left side of the brain.
In ancient Greek mythology, Mercury (or Hermes) was the Messenger God, the one who conveyed messages between the Gods living on Mount Olympus and those living on Earth or in the Underworld. He was the only one well-received in all courts. How did he do it? He had to tailor his opinions and words. He was flexible in his views.
In present-day terms he might be referred to as two-faced. Mercury rules over the double-bodied sign of Gemini. But what would the world do without him? Many solutions require the mental flexibility of a go-in-between with a strong mercury.
The planet itself never goes too far behind or too far ahead of the Sun. Oftentimes, the ancient people weren't able to track Mercury's movements well because it was always hidden under the beams of the Sun. They never knew if the next time he becomes visible he will come before or after the Sun. He could turn up both ways. It is hardly a coincidence that the twins in the sign for Gemini (the male domicile of Mercury) look in opposite directions.
Following from Mercury's flexibility is its association with the trickster, with the mental acrobatics required in business dealings and the communicative skills required to haggle at ancient town markets. Mercury rules over trade and merchants.
It also rules over the 1st house of self and identity. The first house is the place where the soul meets the body, where the metaphysical merges with the physical. Again, there is a duality.
Mercury is often associated with the Magician card in Tarot, the card that unites Above and Below.
Gemini people are famous for their sense of humor but what is humor? Where does it come from? What makes one story hilarious and another one boring. Stories are boring if they are predictable. Mercury is not, you never know from which side of the side it will re-appear. Funny stories are funny because mid-way the script is flipped. You thought A event was going on but actually all this time you had been a character in B event. Here again we see two vastly different scenarios being connected.
Mercury has commonalities with Mars in that they both like to cut and dissect. But Mars does so physically, aggressively, passionately and violently. Mercury does so with the powers of the mind, an action often referred to as analysis and articulation. Take for example a novel. If you are to analyze it, you have to break it up by chapters, by character arcs, by storylines. Then examine all parts separately and put them back together to better understand how they feed one another. That's Mercury's take on cutting and dissecting. This approach to life lies at the core of computing, mathematics, logic, engineering, linguistics, education - the very same things that drive civilizational progress forward.