1. Problems for Identity Theory

2. More Serious Problems
Before proceeding, we need to get a bit clearer on the nature of the theory we are criticizing.

3. Tokens and Types
When Smart says that all mental states are brain states he could be referring to tokens of such states or types of such states.

4. Tokens and Types Mississippi
How many letters are there in the following word:
Mississippi
There are two correct answers:
11 (11 letters of any kind)
4 (4 types of letter: “M”, “I”, “S”, “P”)

5. Tokens and Types
A token is a particular thing. A type is a kind of thing.

6. Tokens vs. Types
Using this distinction we can distinguish two different physicalist claims:
Token Physicalism
Type Physicalism

7. Token Physicalism
Token Physicalism: Every particular mental event is identical to some physical event or other.

8. Token Physicalism
Token physicalism is a very weak thesis. For all it says, pain in one case could be C-fibers firing, but later on, pain could be D-fibers or some other neural event.

9. Type Physicalism
By making claims such as: “pain=C-fibers firing” brain identity theorists like Smart clearly mean something stronger than this. What they want to say is that every instance of pain, is an instance of C-fiber firing.

10. Type Physicalism
So brain identity theory is a version of: Type Physicalism: Every mental event type is identical to a physical event type.

11. Type Physicalism
This is a much stronger claim. To show that brain identity theory is false all we need are possible cases in which you have a pain but you lack C-fibers.
This doesn’t work for token physicalism. As long as there is some physical state or other, you are in the clear!

12. Objections to Brain Identity Theory
Modal objection
Multiple realizability

13. Background on Possibility
When philosophers talk about modality they are usually talking about a domain of metaphysics having to do with possibility and necessity.

14. Background on Possibility
Possible World: A way the world might have been that settles every question of fact (for any claim p, either p or not-p is true).

15. Background on Possibility
p is necessary if and only if p is true in every possible world. Necessary truths could not have been false no matter how the world might have been: e.g. 2+2=4.

16. Background on Possibility
p is possible if and only if, there is at least one possible world where p is true. There is a way the world might have been such that p would have been true.

17. Background on Possibility
For example: So there is a possible world in which I wore a red shirt today. There is no possible world in which I (Tim Butzer) was a rock.

18. Background on Possibility
As we have seen before (recall the Amnesia case) it turns out that identity claims (if true) are necessary. So if A=B, there is no possible world where A and B are different things.

19. Background on Possibility
So if you can show that it is possible for A and B to be distinct, you have shown that they are not really the same thing. This is just what Descartes tried to do with the conceivability argument!

20. The Modal Objection
The brain identity theory says that pain=C-fibers firing. So according to this theory, it is impossible to have pain without C-fibers firing and it is impossible to have C-fibers firing without pain.

21. Reading Start reading Fodor: “Special Sciences.”
(up to “I take it that the discussion thus far…”)
116 (starting with: “This brings us to why there are special sciences at all.”)-117

22. A Quick Recap From Friday
Brain Identity theory is a version of type identity theory
Pain is identical to a kind of physical event: C-fibers firing.
There is pain if and only if there are C-fibers firing.

23. A Quick Recap From Friday
If brain identity theory is true, then it must be necessary that pain=C-fibers firing. That is, in every possible world where there are C-fibers firing, there is some pain identical to that event. So if we can find a possible world with C-

24. The Modal Objection
David Chalmers introduced the case of zombies. In this context, a zombie is something that is physically identical to me, but lacks conscious experience. A zombie has C-fiber firing, but no pain.

25. The Modal Objection
If cases like this are possible then, the brain identity theory is false.
The Modal Objection
Zombies are possible.
If zombies are possible, then there is a world in which there are C-fibers firing, but there is no pain.
If A can possibly exist without B existing, then A and B are not identical.
Therefore, pain is not identical to C-fiber firing.

26. The Modal Objection
You can think of this as Descartes’ argument inverted:
Descartes argued that you could have the mental state without the physical state.
Chalmers thinks you can have the physical state (C-fibers firing) without the mental state (pain).

27. Conceivability and Possibility (Again)
It is certainly true that we can conceive of zombies. But we know from objecting to Descartes that conceivability doesn’t entail possibility.

28. Conceivability and Possibility (Again)
So a brain identity theorist could try to argue that zombies are conceivable but not possible.

29. Conceivability and Possibility (Again)
The debate here gets complicated and controversial fast. If zombies are possible, it seems we have a good objection to brain identity theory. But it is highly controversial whether or not zombies are possible.

30. Conceivability and Possibility (Again)
In short, many people take such arguments to be falling into the same trap as Descartes fell into. But if you think that a zombie case is possible, this might be a reason for you to reject brain identity theory.

31. Multiple Realizability
Recall that brain identity theory is a version of type physicalism. That is, the theory claims that every mental event type is identical to a brain event type.

32. Multiple Realizability
The objection from the multiple realizability of the mental argues:
That there is no single brain or physical state type that corresponds, without exception to pain.
So any version of type physicalism is false, including brain identity theory.

33. Octopi and Aliens
“But (brain identity theory) implies that unless an organism has C-fibers, it cannot have pain. But aren’t there pain-capable organisms, like reptiles and mollusks, with nervous systems very different from the human nervous system? Perhaps in these species the neurons…are not like human C-fibers at all.” (121)

34. Octopi and Aliens
An octopus nervous system is very different from our own. It is much more decentralized. They have a brain, but they also have complex information-processing nerve clusters in all of their arms.
Something like

35. Octopi and Aliens
Octopi can clearly feel pain. But it is unlikely that they have any C-fibers in their radically different nervous systems.

36. Octopi and Aliens
Looking around the animal kingdom it seems that we share many mental state types with many different animals.
Fear
Physical attraction
Hunger
Stress
Perceptual experiences (seeing green etc.)

37. Octopi and Aliens
Some more intelligent animals will also have mental states such as:
Beliefs
Desires
Memories

38. Octopi and Aliens
According to brain identity theory, if two creatures share a mental state type such as fear or pain, their brains must be identical in that regard.

39. Octopi and Aliens
But we have no reason to suspect that this is true, and it seems highly improbable that it will turn out to be right for every mental state type across every organism.

40. Octopi and Aliens
Even worse: consider possible alien creatures that we have not yet discovered. If we ever find other forms of life not native to earth it is likely that they will be capable of feeling pain.

41. Octopi and Aliens
The brain identity theory is committed to the implausible claim that every single creature that we ever find that feels pain will also have C-fibers.
Life will have evolved in different parts of the universe, under different conditions, etc.

42. Octopi and Aliens Still worse: We are carbon-based life-forms.
Silicon is structurally very similar to carbon.
It is generally acknowledged to be possible for life to be silicon based.
But C-fibers are (partially) composed of carbon.
Therefore any silicon based life-form will not have C-fibers

43. Octopi and Aliens
The brain identity theory is committed to saying that any such life-form is incapable of feeling pain. What an amazing claim to make from the armchair!

44. Multiple Realizability
What all of this seems to show is that mental states like pain are multiply realizable. That is, they can be realized by many different physical/brain structures.

45. Multiple Realizability
The multiple realizability of mental states entails that no type physicalist theory of the mental can be correct.

46. Multiple Realizability
But what about token physicalism? Can it still be true that each mental state is identical to some physical state or other? Multiple realizability does entail that this sort of view is false.