A knife in a dream can point to a threat you sense but haven't named, or to a truth that feels sharp enough to cut through pretense. Holding the knife yourself often reflects a decision to cut ties with something, while facing one aimed at you may point to feeling exposed or attacked.
You can address Mercury on the empty signs while the Moon can be addressed on both positions indistinctively. - Picatrix (Andalusia, ~1000.AD)