 It may appear obvious that premise P1 (Mary has complete physical knowledge about human color vision) implies C1 (Mary knows all the physical facts about human color vision). If all physical facts can be known under some physical conceptualization, then a person who has complete physical knowledge about a topic knows all the relevant physical facts. But a few philosophers can be understood as objecting against precisely this apparently unproblematic step. Harman (1990) argues that Mary does not know all the functional facts concerning human color vision because she lacks the concept of what it is for an object to be red, blue, etc. Flanagan (1992) distinguishes metaphysical physicalism from linguistic physicalism. While metaphysical physicalism is the ontological claim that there are no non-physical individuals, properties or relations and no non-physical facts, linguistic physicalism says that “everything physical can be expressed or captured in the languages of the physical sciences.” According to Flanagan Mary’s case may refute linguistic physicalism but does not refute metaphysical physicalism. Alter (1998) points out that the knowledge argument needs the premise that all physical facts can be learned discursively and argues that this assumption has not been established. It may be argued against this view that it becomes hard to understand what it is for a property or a fact to be physical once we drop the assumption that physical properties and physical facts are just those properties and facts that can be expressed in physical terminology.