Changing places
Encyclopedia
The changing places thought experiment
Thought experiment
A thought experiment or Gedankenexperiment considers some hypothesis, theory, or principle for the purpose of thinking through its consequences...

 was conceived of by Max Velmans
Max Velmans
Max Velmans is a Professor of Psychology at Goldsmiths, University of London. From 2003 to 2006 he was Chair of the Consciousness and Experiential Psychology Section of the British Psychological Society and has been involved in many other initiatives to encourage studies in consciousness...

, Professor of Psychology
Psychology
Psychology is the study of the mind and behavior. Its immediate goal is to understand individuals and groups by both establishing general principles and researching specific cases. For many, the ultimate goal of psychology is to benefit society...

 at Goldsmiths College, University of London
University of London
-20th century:Shortly after 6 Burlington Gardens was vacated, the University went through a period of rapid expansion. Bedford College, Royal Holloway and the London School of Economics all joined in 1900, Regent's Park College, which had affiliated in 1841 became an official divinity school of the...

, and was discussed in his 2000 work, Understanding Consciousness
Understanding Consciousness
Understanding Consciousness is a scientific-philosophical text written by psychologist Max Velmans. The book combines scientific studies of consciousness with philosophy of mind...

. The experiment was designed to demonstrate the difficulties in distinguishing phenomenologically between a first-person experience of an event (a subjective experience of an object) and a third-person experience of the same (that is, the observation of such an experience in a subject).

The experiment

Velmans conceives of a situation in which an experimenter ("E") is observing a subject ("S") exposed to a light stimulus. The differences between the two viewpoints, Velmans argues, is primarily derived from a difference in interest, reflected in a difference in their required activities. To explain, during the experiment S is required only to report on her experiences of the light, which she needs to communicate to E in an appropriate manner. E, on the other hand, is interested primarily in S's experience of the light, and thus E's focus is not just on the light (which he now thinks of as a "stimulus") but also on the observable events in S's brain, and on S's reports concerning what she experiences. Thus, E is interested first and foremost in the subject's experience, and how these relate to the light stimulus and brain states of S that he can observe. In such a case, E's experience of events would be considered "objective" or "public", while S's experiences are "subjective" and "private"; while E's focus is on recording the neural causes and correlates of visual experiences, S is interested only in reporting about such experiences.

However, Velmans points out that all that would be required for S and E to exchange roles is for them to change their respective foci (as he puts it "S and E merely have to turn their heads"), so that E focuses exclusively on the light and reports his experiences, while S focuses her attention not just on the light, but on the events of E's brain and his reports of the experience. In such an event, S becomes the experimenter and E becomes the subject; thus, following current conventions, "S would now be entitled to think of her observations (of the light and E's brain) as 'public and objective' and to regard E's experiences of light as 'private and subjective'".

Velmans points out that this outcome is patently absurd, as the phenomenology of the light (that is, the way that it is experienced) remains the same from the perspective of S or E, whether it is thought of as being an observed stimulus or a subjective experience. Nothing has changed in the nature of the light that either party can observe save in the contextualising focus of their interests. That is, Velmans concludes, there is no phenomenological difference between publicly observed phenomena and private, subjective experiences.
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