 In the assumption that Mary has all physical knowledge (first version) or knows all the physical facts (second version) “physical” is meant in a very broad sense that includes knowledge about (or facts concerning) the functioning of the receptors and neurons involved in color vision (biological and physiological knowledge/facts) as well as knowledge about (or facts concerning) the whole network of causal relations between processes underlying color vision, external stimuli and behavior (functional knowledge/ functional facts). “Physical” knowledge in the broad sense at issue even includes psychological knowledge (e.g. knowledge about the result of psychophysical experiments) in so far as they can be formulated without use of phenomenal terminology. One might try to explicate “physical knowledge” in the sense at issue in roughly the following way: physical knowledge includes all knowledge that is expressible in a terminology that does not contain irreducibly mental terms. It would be natural to define physical facts as those facts that can be expressed in this way. But note that this definition of ‘physical facts’ begs the question against an objection that has been raised against the knowledge argument (see  Section 4.2  below). It is certainly not easy to formulate a precise, adequate and non question-begging account of “physical knowledge” and “physical facts” suited for the discussion of the knowledge argument. It is, however, quite common to assume that our intuitive understanding of “physical knowledge” in the broad sense at issue is clear enough for the purposes of the debate, though some argue that talk of “physical facts” needs clarification (see Alter 1998).