Intelligence in this scheme is narrower than mind. Where mind covers everything a person's attention does, intelligence is the rational faculty proper, the part Ptolemy assigns to Mercury alone, and this entry exists to keep that distinction visible.
Mercury is the smallest and fastest of the seven classical planets, never far from the Sun, and the tradition reads all of that as significant. It has no fixed nature of its own. It is masculine with masculine planets and feminine with feminine, diurnal when it rises before the Sun and nocturnal when it sets after it, benefic with benefics and malefic with malefics. No other planet is described this way. Mercury is the faculty that takes the shape of whatever it works on, which is a reasonable image of reason itself.
That adaptability is also why Mercury is the hardest planet to judge in isolation. A reading has to establish what Mercury is with before it can say what Mercury is. Mercury with Saturn gives a slow, deep and retentive intelligence; with Mars a sharp and combative one; with Jupiter a broad and doctrinal one; with Venus a graceful one; with the Moon a fluent and impressionable one.
The sign matters in the ordinary way. Mercury rules Gemini and Virgo and is exalted in Virgo, and the tradition counts it weakest in Sagittarius and Pisces, the signs opposite those.