Monday, 25 February 2008

Refutation of Mary's room thought experiment

white square

Fix your gaze on the small mark in the center of the image, from a distance of about 25 cm or 10 inches. Keep fixing for two minutes as steadily as possible. Close your eyes and cover them with your palms. Watch the development of after images in the dark for about two minutes. Explanation: See below.


Are there such things as qualia? Those properties of things that cannot be described in physical terms, for instance the perceived "redness" of a tomato? The impression of "red" inside of us cannot be described by reflected wavelengths nor by monitoring the electrical activity of retina and brain cells.

I agree that the subjective impression of redness is something fundamentally different from anything that can be objectively measured. Yet I have my doubts whether the term qualia is useful at all. Its existence isn't falsifiable, therefore it cannot be of any use in the scientific understanding of the world.

Mary's room

As a defense of the qualia concept, several thought experiments have been designed. One of them is Mary's room, proposed 1982 by Frank Johnson:
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 cords 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?
A refutation

As a brilliant scientist, Mary, in her black and white room, certainly would have performed the experiment above, gazing at a black and white pattern. Closing her eyes, she would have perceived first a dark red square surrounded by very light green as an after-image in the dark. Later, yellow and blue sensations can be perceived. From her perfect knowledge of retina cells, she would know that the delay in sensitivity is different for rods and for the cones tuned to different colours. Thus, after being over- or understimulated by white light or by dark areas, the retina cells would produce coloured after-images in a certain order. She would figure out that the dark red after-image square is caused by the same wavelength as is reflected by a tomato in a dark shadow. Thus, before ever stepping out of her room, she would have a perfect knowledge of the redness of a tomato.


Mary would learn nothing in the world outside what she didn't know already in her black and white room. Therefore, this experiment doesn't say anything about qualia.

It is no viable objection to state that Mary shouldn't be allowed to perform certain experiments. Every limitation would destroy the thought experiment as such and would make its conclusion meaningless.

On the other hand, Mary must have a healthy retina, capable of colour vision, because she needs it when stepping out of her black and white room.

I have not been a qualia fan before I tested the experiment that I would propose to Mary. Now, after having seen the outcome, I am even less so.


Wayfarer said...

All this does is show that the experiment is poorly designed, not that qualia don't exist. The experiment aims to show that, in the absence of certain stimuli, a person is not capable of knowing all there is to know about the process of experiencing those stimuli. One could have Mary be deaf and know everything physical about sounds and their effects on the human brain, and then suddenly have her hearing restored and learn some knew fact about sounds. Or Mary could be color-blind, deprived of the taste of pears, etc. Such thought experiments would be essentially the same and not be objectionable in the way you espouse. Your objection is petty.

Christian said...

@ Wayfarer: Pettiness is a virtue in science. So I take your judgment as a compliment. All I want to show is the poor design of the thought experiment. Nothing more, as every careful reader of my post will find out. In thought experiments, much more freedom is allowed than in hard science. But sloppiness is not.