Mercury in Jyotisha is Budha, from the root for 'to know, to discern' — it governs the part of the mind that distinguishes and transmits: language, learning, calculation, commerce, humour, dexterity of hand. It's the most neutral of the nine: it takes on the nature of whatever it's with, which is why the tradition says Mercury 'changes with company'.
Mechanically, Mercury is at home in Gemini and Virgo, exalted in Virgo and debilitated in Pisces — which describes it precisely: Virgo is analysis and detail, where Mercury is clearest; Pisces is dissolution and wholeness, where it can't tell things apart. It's the planet nearest the Sun, so in many charts Mercury shares the Sun's sign — how you express yourself and who you are are often one thing.
Where you'll recognise it: liking things stated clearly, learning fast, restlessness, a feel for words and numbers, seeing both sides at once. Running well, Mercury is your most agile part: communicating, trading, translating, making the complex simple. Stuck, it's scattered or slippery — a little of everything, deep in nothing; or technique in place of truth. Mercury's lesson is that after discerning you have to choose: telling things apart is only the start; saying where you stand is using it.
