Debriefing Jackson

1. What is the main argument?

Premise 1: If physicalism is true, then Mary knows absolutely everything.
Premise 2: It is false that Mary knows absolutely everything.
Conclusion: Physicalism is false.

First, notice the argument is both deductive and valid. A valid argument has a structure such that if the premises are true, then conclusion must be true.

2. Are there any terms that need defining?
In this argument there were at least two.

physicalism
Leibniz’s Law

Remember, Jackson indicated that there could be other versions of physicalism. The way that Jackson uses that term could be a point of contention.

3. Are there objections to the reasoning or premises?
The form, as I outlined above, is good. So, there is no obvious problem. However, notice that Churchland found a technical problem with the term ‘knows about.’ Jackson took this seriously enough to reply. And this seems to be a threat to the force of the argument; that is, that the conclusion that physicalism is false might not follow. Thus, it might be worthwhile to find Churchland’s argument (Jackson lists it in his actual article) and try to understand Churchland’s full complaint. Maybe you’ll end up agreeing with Churchland.

4. What is the issue?
Whether physicalism is true.

5. Does the author present counter arguments?
Yes. He presents at least one by Paul Churchland, as reprinted here. We do not have the space to present the author’s reply, and you’ll need to check the original Churchland to see if the author accurately represents it and adequately responds. Even though, if you were to read the full article, the author’s response might be convincing, remember that he is objecting to Churchland’s characterization of his argument. A nice writing topic might be to defend Churchland’s characterization from Jackson’s reply.

6. Are the premises well supported?
Well, a lot hinges on accepting the thought experiment. It is not a fair criticism to complain that a person with complete knowledge of the entire physical world could not exist. The thought experiment is meant to articulate a commitment that a physicalist would have. Specifically, that the imaginary person, Mary, as stipulated, would know everything there is to know about the physical world without ever seeing color. You might wonder if the physicalist would actually be forced to have that commitment. You might then take issue with Jackson’s definition of physicalism.

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