Jackson -- A Critical Reaction
Jackson, in his article “What Mary Didn't Know” makes the claim that a person (e.g., Mary) existing for her entire life in a black and white world could, by learning all factual knowledge (i.e., in books) can know what it is to see colour. I do not believe that the kind of factual knowldege, of which Mary is described as having in totality, is in any way sufficient for knowledge as a whole or even knowledge of the colour world (i.e., I do not agree that by knowing every physical fact one knows all there is to know).
Jackson states in an endnote that “...if physicalism is true, then if you know everything expressed or expressible in explicitly phsycial language, you know everything” (570). I think this train of thought is fallacious and is the root of the problem with his reasoning. The problem is best explained in Churchland's objection regarding the equivocation in Jackson's simplified argument (568).
The distinction between 'knowledge by description' and 'knowledge by acquaintance' (knowledge-that and knowledge-how) explains what it is that Mary learns upon her excursion into the world of colour.l There are obviously some things that we know which are not expressible in words (i.e., language). For example knowing how to tie one's shoe, or knowing how to ski, or knowing how to shoot a basketball into a hoop. No one can give you a set of instructions which, after reading, will give you the ability to ski or sink a basket. These sorts of things are acquired knowledge. That is, you know these things after sufficient alterations to your neural make-up have been achieved. And these changes only occur when actually experiencing and learning by trial and error until the desired effect has been gained. Similarly Mary cannot know colour (i.e., know how to experience colour) unless she actually experiences it and gains neural connections which make this experience realizable.
So, it would seem, knowledge-that is the sort of knowledge that is conscious and able to be expressed through language (the only kind of knowledge physicalism, as used by Jackson, considers), whereas knowledge-how is unconscious and not linguistic. Knowledge-that can be learned by association, but knowledge-how is learned by basic reordering of neural pathways (i.e., it cannot be associated with anything; it is a wholly new, axiomatic phenomenon to be learned).
Besides the problem that knowing everything (even, only in Mary's world) would require a god-like intellect (i.e., one that is infinite), if Mary existed for her entire life in this world, I do not believe that she would be capable of expreriencing colour even when introduced to our world. If she did not develop a colour faculty as her brain matured then she would lack the fundamental ability to even process what colour is like! In the same way people who are blind from birth and, through some sor tof surgery, are given sight do not understand what their eyes are telling them because their brains have not developed to be able to process this kind of information. Similarly with children brought up without any (or even impoverished) exposure to language, after development, they do not have the capacity to understand and think in a linguistic fashion. The brain centers just aren't there. Mary's brain just wouldn't have the colour processing ability that people who grew up in the colour world would have.
Because the kind of knowledge attributed to Mary is only linguistic/descriptive, because she could not learn knowledge how without actually experiencing it, and because she just wouldn't have the kind of brain actually experiencing it, and because she just wouldn't have the kind of brain thatcould experience colour, Mary didn't know all there was to know in the first place! Therefore, thek ind of physicalism described by Jackson is false (i.e., knoledge-that does not entail knowledge-how), for it is not sufficient for knowledge as a whole.
Works Cited
Jackson, Frank. What Mary Didn't Know, The Journal of Philosophy, Vol. 83, No. 5. (May, 1986), pp. 291-295.