@Physicalism is the thesis that everything is physical, or as contemporary philosophers sometimes put it, that everything supervenes on the physical. The thesis is usually intended as a metaphysical thesis, parallel to the ancient Greek philosopher Thales's thesis that everything is water, or the idealism of the 18th Century philosopher Berkeley, that everything is mental. The general idea is that the nature of the actual world (i.e. the universe and everything in it) conforms to a certain condition, the condition of being physical. Of course, physicalists don't deny that the world might contain many items that at first glance don't seem physical -- items of a biological, or psychological, or moral, or social nature. But they insist nevertheless that at the end of the day such items are wholly physical. stanford:physicalism@

Jackson's knowledge argument against physicalism

Jackson (1982) presents his argument against physicalism using two thought-experiments.

  • Fred is supposed to be able see two different shades of red when we can only see one.
  • Mary the scientist has only seen black and white. But she is supposed to know all the physical facts there are. She is supposed not to know what it is like for others to see colours.

We shall focus on the second argument.

@Mary is a brilliant scientist who is, for whatever reason, forced to investigate the world from a black and white room via a black and white television monitor. She specializes in the neurophysiology of vision and acquires, let us suppose, all the physical information there is to obtain about what goes on when we see ripe tomatoes, or the sky, and use terms like ‘red’, ‘blue’, and so on. She discovers, for example, just which wavelength combinations from the sky stimulate the retina, and exactly how this produces via the central nervous system the contraction of the vocal chords and expulsion of air from the lungs that results in the uttering of the sentence ‘The sky is blue’. (…) 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 is it 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.@

  1. While still in the room, Mary knows all the physical facts there are about other people and their experiences.
  2. When Mary leaves the room and sees something red for the first time, she acquires new knowledge about the experiences of other people.
  3. So when she was still in the room, there was some fact about other people's experiences that Mary did not know.
  4. This new fact about other people's experiences that Mary was ignorant of was not captured by the physical information about those other people. It is a non-physical fact.

Possible replies

Against 1

  • The argument assumes that all physical facts can be learnt in the black-and-white environment.
  • Perhaps there are physical facts that are necessarily subjective?
  • "It is now 3pm." is not reducible to any set of non-indexical statements, but presumably it still describes a physical fact. If there are indexical physical facts, why not subjective physical facts?
  • However, why should we say that these subjective facts are physical facts? The same subjective facts might obtain even if idealism is true.
  • Readings

Against 2

  • The claim that Mary does not know everything there is to know about the colour experiences of other people is an intuition that has no justification.
  • It is hard to imagine what it is like to really know all the physical facts about other people's experiences. It is just as sensible to say that Mary would not learn anything new.
  • See Daniel Dennett's "What Robo Mary Knows":

@In an attempt to bring out the flaws in the thought experiment, I encouraged people to consider a variant ending: And so, one day, Mary’s captors decided it was time for her to see colors. As a trick, they prepared a bright blue banana to present as her first color experience ever. Mary took one look at it and said “Hey! You tried to trick me! Bananas are yellow, but this one is blue!” Her captors were dumfounded. How did she do it? “Simple,” she replied. “You have to remember that I know everything–absolutely everything–that could ever be known about the physical causes and effects of color vision. So of course before you brought the banana in, I had already written down, in exquisite detail, exactly what physical impression a yellow object or a blue object (or a green object, etc.) would make on my nervous system. So I already knew exactly what thoughts I would have ... . I was not in the slightest surprised by my experience of blue (what surprised me was that you would try such a second-rate trick on me). I realize it is hard for you to imagine that I could know so much about my reactive dispositions that the way blue affected me came as no surprise. Of course it’s hard for you to imagine. It’s hard for anyone to imagine the consequences of someone knowing absolutely everything physical about anything!”@

Against 3

  • (Lewis) Agree : Knowing what an experience is like requires having had the experience. Physical knowledge is not enough to give you the experience.
  • Disagree : Coming to know what an experience is like is not a matter of learning new information, or learning a new fact.
  • Justification : Acquiring information, including non-physical information, is not enough to enable Mary to know what it is like to see red.
  • Proposal : Knowing what an experience is like is knowing-how - having the ability to imagine, remember, recognize the experience.
  • Implication : Although Mary learns something new, she acquires new abilities but not any new information.
  • See ConsciousnessAbilityHypothesis

Against 4

  • Mary does learn a new fact, but it is just an old fact under a new "mode of presentation". There are no non-physical objects or properties involved. (Churchland)
  • The H2O analogy : One can know that rivers contain water without knowing that rivers contain (liquid) H2O. Under a fine-grained notion of fact, the fact that rivers contain water is distinct from the fact that rivers contain H2O. So knowing a new fact does not imply knowing about some new object or property.
  • Problem with the H2O analogy - to know that rivers contain water but not that rivers contain H2O is to lack certain physical knowledge, e.g. water molecules are constituted by hydrogen and oxygen. If one knows everything physical there is to know about water, one would know that water is H2O.
  • But the analogy shows that even though fact X ≠ fact Y, Y need not introduce any new entities that are not entailed by X. So even if Mary does learn something new, it does not follow that there is some entity in the world that is not ultimately physical in nature.

Discussion questions

  • What exactly do we mean by "physical"? Is the criterion to be determined by physics? Should it be current physics or future physics? Is the criterion to be determined by some other conception of physical objects?
  • Is Jackson's argument pointless if we do not have a precise definition of "physicalism"?

See Stoljar, Daniel. Physicalism. Stanford Encyclopedia of Philosophy. stanford:physicalism