ESP is an abbreviation for Extra-Sensory Perception. ESP means perception which occurs independently of sight, hearing, or other senses, and is sometimes referred to as the "sixth sense." An amulet of ESP grants extrinsic telepathy when worn, which will show all monsters with brains within a 10x10 grid. If you polymorph into a metallivore and eat the amulet, it will instead grant intrinsic telepathy. This takes effect only when you are blind, and shows all monsters with brains on the whole level.