Why does Mary learn something when she leaves the room? One answer, endorsed by some physicalists as well as most dualists, is as follows. Mary learns something because phenomenal knowledge requires direct acquaintance with phenomenal properties. For this reason, there is an epistemic gap between the physical and the phenomenal: phenomenal facts cannot be deduced from physical facts. This is the acquaintance response to the Knowledge Argument. The physicalist and dualist versions of the acquaintance response diverge as to whether this epistemic gap reveals an ontological gap between the physical and the phenomenal.