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From “What Mary Didn’t Know” by Frank Jackson:
Mary is confined to a black-and-white room, is educated through black-and-white books and through lectures relayed on black-and-white television. In this way she learns everything there is to know about the physical nature of the world. She knows all the physical facts about us and our environment, in a wide sense of ‘physical’ which includes everything in completed physics, chemistry, and neurophysiology, and all there is to know about the causal and relational facts consequent upon all this, including of course functional roles. If physicalism is true, she knows all there is to know. For to suppose otherwise is to suppose that there is more to know than every physical fact, and this is just what physicalism denies.
Physicalism is not the noncontroversial thesis that the actual world is largely physical, but the challenging thesis that it is entirely physical. This is why physicalists must hold that complete physical knowledge is complete knowledge simpliciter….
It seems, however, that Mary does not know all there is to know. For when she is let out of the black-and-white room or given a color television, she will learn what it is like to see something red, say. This is rightly described as learning – she will not say “ho, hum.” Hence, physicalism is false. This is the knowledge argument against physicalism in one of its manifestations. This note is a reply to three objections to it mounted by Paul M. Churchland….
Churchland’s first objection is that the knowledge argument contains a defect that “is simplicity itself.” The argument equivocates on the sense of ‘knows about’. How so? Churchland suggests that the following is “a conveniently tightened version” of the knowledge argument.
Therefore, by Leibniz’s law,
Churchland observes, plausibly enough, that the type or kind of knowledge involved in premise 1 is distinct from the kind of knowledge involved in premise 2. We might follow his lead and tag the first ‘knowledge by description’, and the second ‘knowledge by aquaintance’; but, whatever the tags, he is right that the displayed argument involves a highly dubious use of Leibniz’s law.
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