Mary and her Sister: debunking a classic thought experiment

Some years ago I thought up an alternative version of Frank Jackson’s well-known thought experiment concerning Mary, the super-scientist who knows all the facts about the human visual system, the physics of light and colour, and so on, but has been brought-up in a black-and-white room and is therefore supposed to learn some new fact when she first sees a coloured object. I still have a soft spot for my version of Jackson’s story, at least as an intuition pump, and thought it worth bringing to the light of day.[1]


Mary has a twin-sister, Marianne, and both of them have been brought up in a black-and-white apartment by a group of scientific researchers. Unlike her sister, Marianne has not been educated in the sciences but has developed into an extremely talented musician. Neither sister has ever seen a colour – just black, white, and various shades of gray.

One day a member of the research team asks Marianne to take a break from her piano practice and presents her with a sheet of coloured paper. After Marianne has overcome her astonishment, the researcher asks her to describe her experience:

“Well, I was quite surprised at first, and a bit puzzled, but I guess that’s the effect of seeing something unexpected and new. I suppose I must be experiencing one of those ‘colours’ that my sister is always talking about. As for the colour itself, it’s hard for me to describe. I mean, I haven’t seen it before, and it’s unlike anything that I’ve ever experienced. It’s  … um … it’s just different from black, white or gray – that’s the only way I can describe it. Oh, hold on a minute, it seems to be quite dark, like a dark shade of gray … what I mean is, it’s nothing like white at all.”

So the researcher writes down in her report: ‘hard to describe; a colour I haven’t seen before; unlike black, white or gray; but dark like a dark shade of gray, and nothing like white’.

Now the researchers have already told Mary, the super-scientist, all the details of the colour that they are going to show to Marianne. They have asked Mary to predict what it would be like for her sister when she sees precisely that colour, and Mary has placed her prediction inside a sealed envelope. So after the researcher has left Marianne’s room, the envelope is torn open and on the sheet of paper inside is written:

“Hard to describe; a colour I haven’t seen before; unlike black, white, or gray; but dark like a dark shade of gray, and nothing like white.”

So Mary knew exactly what it would be like for her sister to experience seeing that particular colour!


What, then, did I conclude from this little story about Mary and her sister? Note, firstly, that no mention is made of which colour is on the sheet of paper. This avoids the unwarranted assumption on the part of the reader that what Marianne learns on first seeing the colour is along the lines of ‘what it is like to see red’ – which the reader will assume is the same as what it is like for them to experience seeing a red object. I am therefore putting the reader into a similar position, epistemologically speaking, to Marianne.

Secondly, Mary’s extensive knowledge of neuroscience, optics, and colour theory plays no part whatsoever in arriving at her prediction. In the original thought experiment, we are provided with a description of Mary’s knowledge base, and asked whether she could know in advance what it will be like for her when she first sees, say, a red object. So it is natural for the reader to assume that Mary must somehow use her specialized knowledge in order to understand what it would be like for her on the first occasion that she sees something red. In that sense, Jackson’s argument is a classic conjuring trick: the audience’s attention is misdirected towards Mary’s specialised knowledge, and away from the phenomenology. But if we describe the thought experiment correctly, then the phenomenology gives the game away: it’s just ‘a colour I haven’t seen before’.

So the point of my reformulation of Jackson’s thought experiment was to show that the knowledge argument gains a large part of its credibility from an implicit assumption that there is some phenomenal property, presumably an intrinsic property, which is common to all experiences that people have on seeing, say, a red object under normal conditions – a property that is common to my experience, to your experience, and to Marianne’s experience when she first sees such an object.

But this is an unwarranted assumption for an argument that aims to establish the impossibility of materialism – it effectively assumes at the outset what it wants to prove. It seems to me to be perfectly coherent for there to be no common property of the experiences of different people when they observe a red object – no property that is shared by my experience, your experience, and Marianne’s experience. Instead, the phenomenal property of my experience when I see a red object may be something like ‘a colour which I, Mike, recognize as that which I term red’), and this differs from the property of your experience when you see the same object (‘a colour which I, the Reader, recognize as that which I term red’), and from the property of Marianne’s experience when she observes a red object for the first time in her black-and-white room (‘a colour which I, Marianne, do not recognize’); such phenomenal properties would reflect the subjective nature of the experiences of separate individuals. To express this as a slogan: phenomenal properties should follow the phenomenology.

The alternative is to maintain that phenomenal properties are in some sense objective: that if you, and I, and Marianne in her black-and-white room, all look at some red object, then our experiences will all have the same phenomenal property. This would be to treat having an experience in the same manner as perceiving an external object: for example, if you and I and Marianne were all looking at a mountain, it would be the same mountain we were looking at even if we all perceived it from completely different angles and distances. But the mountain is an external object, and experiences are not.


[1] Section 4(c) of my article ‘The Possibility of Materialism’ sets out a more comprehensive response to Jackson’s thought experiment and the ‘knowledge argument’ against materialism.

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