Jackson quotes are from "Epiphenomenal Qualia."

Jackson describes himself as "a qualia freak". The word "qualia" is the plural of the word "quale" (pronounced (KWA-lay). A quale is a "raw feel". Examples of qualia include "the hurtfulness of pains, the itchiness of itches, pangs of jealousy" and the taste of pineapple, the smell of a rose, etc.

Some people think qualia are the obvious counterexample to physicalism's claim that everything about the mental can be understood in purely physical terms. Qualia are what physicalism leaves out. You could know everything about the physiology of smell and the chemistry of pineapple esters, etc., – i.e., you could know all the physical facts – and still miss the quale of pineapple taste.

Jackson's knowledge argument against physicalism is well-known in philosophy. He asks us to perform a couple of thought experiments. We think about the fictional Fred, who can see colors that we cannot.

"What kind of experience does Fred have when he sees red1 and red2? What is the new color or colors like? We would dearly like to know but do not; and it seems that no amount of physical information about Fred's brain and optical system tells us. We find out perhaps that Fred's cones respond differentially to certain light waves in the red section of the spectrum that make no difference to ours (or perhaps he has an extra cone) and that this leads in Fred to a wider range of those brain states responsible for visual discriminatory behavior. But none of this tells us what we really want to know about his color experience. There is something about it we don't know. But we know, we may suppose, everything about Fred's body, his behavior and dispositions to behavior and about his internal physiology, and everything about his history and relation to others that can be given in physical accounts of persons. We have all the physical information. Therefore, knowing all this is not knowing everything about Fred. It follows that Physicalism leaves something out."

Jackson then asks us to think about the scientist Mary, who knows everything there is to know (every physical fact, that is) about color vision, but has never seen red.

"What will happen when Mary is released from her black and white room or is given a color television monitor? Will she learn anything or not? It seems just obvious that she will learn something about the world and our visual experience of it. But then it is inescapable that her previous knowledge was incomplete. But she had all the physical information. Ergo there is more to have than that, and Physicalism is false."

Philosophers Saul Kripke and Thomas Nagel have also given arguments against physicalism. Kripke's argument is called the modal argument. An account of that argument is below. Nagel's argument is the one you have read in "What is it like to be a bat?"

Jackson rejects both arguments.

After reading "Epiphenomenal Qualia", try to formulate precisely why Jackson disagrees with both Kripke and Nagel.

Kripke's Modal Argument

In the 1970's Saul Kripke gave an influential argument against physicalism: the so-called modal argument. The word "modal" is this context refers to a specialized branch of logic concerned with questions of possibility and necessity. You don't have to know much about modal logic to understand Kripke's argument, however.

The fundamental insight is that if A is identical to B, then A is necessarily identical to B. In other words, if A is identical to B, A is identical to B in every possible world: it is impossible for A not to be B. Philosopher David Papineau explains Kripke's argument as follows:

"It may be a matter of empirical discovery to find out that Cicero is identical with Tully, say, or that Evening Star with the Morning Star, or temperature with mean kinetic energy. But the truths so discovered are necessary truths. To suppose otherwise is to suppose that Cicero might not have been Tully (or the Evening Star might not have been the Morning Star, or temperature might not have been kinetic energy). However, these things make no sense. How could Cicero not have been Tully? There is no possible world in which Cicero exists, but not Tully. Since they are the same person, Tully will be there if Cicero is....

"So identities are necessary, if true. In particular, mind-brain identities would have to necessary if they were true.

"So far this mightn't look like much of an objection to materialism. For why can't materialists simply ... agree that mind-brain identities are necessary, while based on empirical evidence?...

"The trouble now, however, is that these identities don't seem necessary at all. The necessity of Tully=Cicero shows itself in the fact that a world containing Tully but no Cicero makes no sense. But there doesn't seem anything similarly incoherent about a world with pains but no brains [as a ghost might feel], or brains with no pains [as a zombie might feel]. It seems possible for pains and brains to come apart, in a way that Cicero and Tully simply can't." (p 18-19 of Papineau's book-in-progress The Phenomenon of Consciousness, online here)

So this is Kripke's argument: if the mental were identical to the physical, then we wouldn't be able to imagine mental without physical or physical without mental, any more than we could imagine Cicero without Tully or Tully without Cicero. But we can conceive of mental without physical (e.g., ghosts), and physical without mental (zombies). So, by modus tollens, the mental can't be identical to the physical.

What It Means to Say Qualia are Epiphenomenal

Physicalism assumes that mental events have a genuine causal role. For example, according to the physicalist picture, C-fiber stimulation actually does something in the body; it causes other bodily events. Physicalists also usually assume that qualia function causally. For example, physicalists usually assume that pain has an evolutionary function: pain teaches organisms to avoid hurtful stimuli.

By contrast, something is epiphenomenal if it cannot function causally in the world. Jackson argues that qualia are epiphenomenal; i.e., they don't cause anything physical (he allows that they might cause other mental events) and furthermore, that "their possession or absence makes no difference to the physical world."

If Jackson is right, and qualia are epiphenomenal, there is at least one class of mental states that do not function causally. See what you make of Jackson's arguments for epiphenomenal qualia!