Jackson’s argument can be reconstructed like this:
Jackson thinks that qualia, or phenomenal properties of mental states are causally inefficacious: their possession or absence makes no difference to the physical world. However, he thinks that the instantiation of qualia may make a difference to other mental states. Most of his defense will consist in rejecting some reasons to think that qualia are causally efficacious.
The first thing to note is that, the way Jackson originally stated the argument, there is an ambiguity in the notion of information. In one sense, ‘Hesperus is a planet’ and ‘Phosphorus is a planet’ transmit different information, since someone may know that Hesperus is a planet but still not know that Phosphorus is a planet. In the other sense, ‘Hesperus is a planet’ and ‘Phosphorus is a planet’ give us the same information, since they express the same proposition, or are true under exactly the same circumstances. Which sense of information is the one required by the argument? Let’s examine both.
For the sake of clarity, let’s call the first notion of information narrow and the second wide.
With narrow information, we may present the argument as follows:
Here the conclusion is to be understood in the following terms: that one can know something when described in a particular way, but not under a different way. This is just like knowing that Hesperus is a planet but not knowing that Phosphorus is a planet.
This sort of conclusion need not worry the physicalist. Physicalism is usually meant as a metaphysical thesis about the nature of the mental. In its simpler formulation, it claims that mental things are physical things, or reducible to physical things. This metaphysical claim is compatible with the claim that mental vocabulary may just not be translatable to physical vocabulary, or that mental concepts are not reducible to physical concepts.
However, there is a stronger version of the argument, using the wide notion of information. Jackson’s own presentation is as follows:
On this way of understanding the argument, what Mary doesn’t know is a fact. It’s not only that she lacks the appropriate concepts, or that she doesn’t know under a particular description. What she fails to know is a fact, and given that she knew all the physical facts, this new fact can’t be physical. Questions: What do you think of this version of the argument? Has Jackson made a compelling case that the main premise is true?