A technician explains measurements whilst a scientist explains observations.

Zombies and progressive replacement of the brain

Pylyshyn (1980) proposed a thought experiment in which he asked what would happen if the physical brain were progressively replaced with components that performed the same functions as those biological components that they replaced. This thought experiment has been used to support types of "functionalism" - the idea that we are the sum of the processes that occur between us and the environment (Chalmers 1996).

What would happen if the brain were replaced with artificial components? Wikimedia commons

What would we expect if the general idea of Pylyshyn's experiment could be put into effect?

At one extreme the experiment could involve the replacement of the brain with organic components that are identical to those that already exist. In this case the new brain would only differ from the old brain in terms of its quantum state and its history. If the mind involves quantum states or history (eg: if it is extended in dimensional time) then the new mind would differ from the old, otherwise the new mind would be the same as the old mind.

At the other extreme the brain might be replaced by computer cards held in racks at various different locations in the world but interconnected by broadband. These cards would be designed to perform, as far as is possible, the same overall processes as the original brain, converting a pattern of input into the same pattern of output as the original organic brain might have produced. This system would differ from the original brain in terms of physical form, internal state and history. If the mind depends upon the form of its content then it would not be replicated if half of the data from a retina were processed in France and another ten percent in Brasilia etc.

It seems that Pylyshyn's "experiment" teaches us nothing that we did not already know - if the spacetime form of the world is of no significance for creating mind then we could be replicated by simple machines like silicon chip based computers. If the spacetime form of the world is significant for the presence of mind then we can only be replicated by conceptually complex machines or cannot be replicated at all.

Considerable discussion has been devoted to whether a progressive replacement of the brain would lead to the slow fading of the content of experience or the disappearance of areas of experience etc (Chalmers 1996). Just asking this question presupposes that we would "see" the content of our brains in a type of Cartesian Theatre and hence suggests that the spatio-temporal form of the mind and brain are important.

Chalmers believes that the progressive replacement would not affect the content of experience. I would agree provided that the replacement were with parts that perfectly replaced both the form and function of the original parts, not just the function of the originals.

Chalmers uses Pylyshyn's "thought experiment" to suggest that if two processors share exactly the same functional architecture then if one is conscious so is the other. Chalmers writes: "I claim that conscious experience arises from fine-grained functional organization. More specifically, I will argue for a principle of organizational invariance, holding that given any system that has conscious experiences, then any system that has the same fine-grained functional organization will have qualitatively identical experiences." (The Conscious Mind). The inclusion of 'fine-grained' means that what Chalmers is really saying is that if two things are identical in form and function they are the same. This argument cannot be used to suggest that a robot that has the same behaviour as a person has the same 'conscious experiences' because the robot does not have the same 'fine grained functional organisation'. If it had exactly the same 'fine grained' functional organisation it would consist of exactly the same macromolecules and materials as a person and would not be a 'robot'. Unfortunately the argument is often referenced to support the possibility of conscious experience in machines that only replicate our overall behaviour. This mistaken interpretation of the argument is often seen in the discussions of "philosophical zombies" in which the possibility that there could be organisms that perform the same functions as humans but have no conscious experience is debated, unfortunately few, if any of these discussions focus on the possible significance of geometrical form in a multidimensional universe.

In conclusion it can be seen that the role of geometrical form and the time extension within the form of our experience is crucial for working out what might happen during replacement of the brain. If our minds depend on extension in time then a progressive replacement of the brain with organic components that are identical to the original would probably lead to some loss of awareness of the past but would otherwise have little effect. If mind depends on time extension (see Time and conscious experience) then replacement with parts that could not duplicate the time extension would lead to the disappearance of conscious experience. Of course, we do not know what sort of components would support a time extended mind, it may be the case that even replacement with organic components that are identical to the original might not work for reasons as yet unknown.


Pylyshyn, Z. (1980) The "causal power" of machines. Behavioral and Brain Sciences 3:442-444.

Chalmers, D. (1996). The Conscious Mind. New York: Oxford University Press.

Marcel AJ. (1998). Blindsight and shape perception: deficit of visual consciousness or visual function? Brain 1998, 121, 1565-1588. http://brain.oupjournals.org/cgi/reprint/121/8/1565.pdf

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