1
Mary's story gives two premises
If physicalism is true, a person who knows all the physical facts has nothing left to learn about what red looks like. And on seeing red, Mary does learn something. The conclusion is that physicalism is false.
Lesson · Mind, Language and Meaning · Mind
The Knowledge Argument
Mary knows every physical fact about colour and has never seen red. Formalise the argument that she learns something on release, show it valid as modus tollens, and show that the physicalist's replies deny a premise or charge equivocation, and do not dispute the validity.
a: physicalism n: Mary learns a fact
a → ¬n, n ∴ ¬a
valid: no row has true premises, false ¬a
reply: deny n (an ability, not a fact)
reply: two senses of knowing, two atoms
A valid argument, a denied premise
Core ideas
What this lesson establishes
The knowledge argument is short, and the temptation is to wave it away. The lab shows where a reply has to go instead.
2
Validity is the first question, and the lab answers it
The form is modus tollens, which has no counterexample row. So the physicalist must deny a premise, and the lesson is about which, and at what cost.
3
Changing an atom changes the argument
If the premise about a new fact is replaced by one about a new ability, or if “knows” is used in two senses, the argument is no longer the same argument. The lab shows this as a counterexample row.
The material
Mary, in three presets
The argument as stated, the reply that changes an atom, and the charge of equivocation.
Mary is a scientist who knows every physical fact there is to know about colour and the vision of colour. She has lived her whole life in a room where everything is black, white or grey. One day she is let out and sees a ripe tomato. The knowledge argument says that she learns what red looks like, that this is something she did not already know, and that therefore not every fact is a physical fact.
Put it in standard form. Let a say that physicalism is true: every fact is a physical fact. Let n say that on release Mary learns a new fact.
- If physicalism is true, Mary, who already knew every physical fact, learns no new fact: a → ¬n.
- Mary does learn a new fact on release: n.
- So physicalism is false: ¬a.
This is modus tollens. The lab builds a row for every assignment of a and n, four in all, and finds none in which both premises are true and the conclusion false. The verdict is valid, and the form is named. That settles the logic, and puts all the weight on the premises.
Definition · The physicalist's two ways out
A physicalist can deny a premise, most naturally the second, or can say that the argument equivocates, so that its two premises do not share the sense they need. Neither is a claim that the form is invalid.
The first is the ability reply. What Mary gains is not a fact but an ability: to recognise red, to imagine it, to remember it. Knowing how is not knowing that, and a new ability is not a new fact. This denies n. The second preset shows what happens when the argument is restated with the ability in the place of the fact: let k say that Mary gains a new ability. Then a → ¬n and k do not lead to ¬a, and the lab finds a counterexample row, with a and k true and n false. A new ability does not conflict with physicalism. The reply is not that the argument is invalid. It is that the fact premise is false, and that the ability premise, which is true, does not carry the conclusion.
The second is the charge of equivocation. Knowing a physical fact is knowing that something is so; knowing what red looks like seems to be knowing it by acquaintance. If the two are different senses, then the premise that links them for physicalism fails. In the third preset the senses are separate atoms: f for Mary's knowing all the physical facts, l for her lacking what red looks like in the same sense, and m for her lacking it in the sense of acquaintance. The premises are a → ¬(f ∧ l), f and m. The lab finds a counterexample row, with a, f and m true and l false.
That is the physicalist's point, and the dualist's answer is also visible. Make l and m one atom, so that what Mary lacks is the same in both premises, and the lab finds the argument valid again. Whether they are one atom is what the dispute is: the dualist holds that acquaintance with red is knowledge of a fact, and the physicalist that it is not.
Each side has a cost. To deny that Mary learns a new fact is to say that her coming to know what red looks like adds nothing to what is so, which many find hard to say with a straight face. To accept the new fact is to accept that something is left out of the physical story, which is what the physicalist cannot. The lab checks the logic and leaves the costs for you to weigh.
Interactive
Validity by cases
Every row, every counterexample, and the form
Method
Formalising a philosophical argument
Five steps. The third is where replies get their grip.
1
Pick one letter for each claim
Physicalism, Mary learns a new fact, and so on. Two letters for two claims, and no letter for a claim that is used in two senses.
2
Write each premise and the conclusion
Use the connectives of the earlier course. A conditional for what physicalism says Mary would not learn, and a plain letter for what she does learn.
3
Check the atoms across the premises
A letter must mean the same in each premise it appears in. If “knows” is used in two senses, give each its own letter and see whether the argument survives.
4
Ask the lab for the verdict and the row
A verdict of valid leaves only the premises to doubt. A verdict of invalid shows the row, and the row says which case was overlooked.
5
Place the reply
Say which premise a reply denies, or which atom it splits. A reply that does neither has not yet met the argument.
Where the reply goes
Score 0 / 0Choose the answer you can justify.
Mary as modus tollens
Premises a → ¬n and n, conclusion ¬a.
a n | a → ¬n n | ¬a
T T | F T | F
T F | T F | F
F T | T T | T
F F | T F | T
Only the third row makes both premises true, and in it the conclusion is true. The argument is valid. The ability reply goes after the premise n, and the equivocation reply after the match between the senses; neither touches the table.
Common mistakes
1
Thinking the knowledge argument is invalidThe lab checks it. With a → ¬n and n as premises and ¬a as the conclusion there are four rows and no counterexample, and the form is modus tollens. A physicalist who wants to resist it must deny a premise or charge equivocation, and the replies are good or bad according to what they say about the premises.
2
Counting a change of atom as a refutationThe ability preset has a counterexample row, but it has changed the argument. k is not n. The ability reply does not show that Mary's argument fails to follow; it asks the dualist to defend the claim that Mary learns a fact.
3
Treating the equivocation charge as settledThe third preset shows what the charge would show, if the two senses are different. Make l and m one letter and the argument is valid again. Whether they are one is the question between the sides, and the lab does not decide it.
Check your understanding
✓
Finish when you can place a reply to the argument.
Given the knowledge argument or a reply to it, you should be able to write the premises with one letter for each claim, check the validity in a lab, and say which premise the reply denies or which atom it splits.
Note: The dualist and the physicalist can both read this lesson as favouring them. The lab does not choose, because choosing is a matter of which premise to give up.