It is clear that, first, in all the stages up to this point all reactions are totally mechanical, completely explicable as a cause-and-effect process and, second, that there is no seeing taking place yet. The exploration so far is concerned solely with physical states; no conscious states have emerged. An eye on its own does not ‘see’ any more than a camera does. Indeed, the way in which light is focussed through an adjustable aperture on to light sensitive material is comparable to the workings of a camera. (Or rather the other way round since man-made cameras are constructed as copies of the natural visual process.) A camera may receive an image but it has no experience of what seeing is, quite unlike the phenomenon of human or animal consciousness.
A rod cell is a minute but extremely complex piece of biological machinery. In length it is three five hundredths to one hundred and twenty fifth of an inch. At the end facing the front of the eye and so receiving the light, each rod cell contains 2000 discs lined up behind one another and containing 100 million molecules of a light-sensitive pigment called rhodopsin. In a thousandth of a second light transforms rhodopsin to metarhodopsin, the first in a series of chemical processes which culminate in the transmission by a synapse of signals to dendrites of adjacent cells, and from there eventually to the brain. At this stage the process has no quality that in the remotest sense could be called the conscious state of seeing. At the retina was an image of the object of vision. Now, however, that image has been translated into the completely non-pictorial form of chemical information within cells.
The rods and cells convey their information to retinal ganglion cells which form the optic nerve. This nerve enters the skull through a gap called the optic foramen. At a junction in the brain, the optic chiasma, fibres from the left side of the right eye join fibres from the left side of the left eye. The same happens with the right side of the eyes. On each side of the brain an optic nerve passes along an optic tract, that is, the one on the left-hand side of the brain with information from the left sides of the two eyes, the one on the right-hand side of the brain with information from the right sides of the eyes. They reach a lateral geniculate nucleus, one on each side of the brain and from there to the primary visual cortex at the back of the brain where another series of processes takes place: the sorting of the visual data, the transmission of part of this information to other specialised regions nearby in the brain, the analysis and comparison of colours and contrasts. So demanding are the processes of vision that they occupy one fifth of the brain’s capacity. Even at this point in tracing out the process the vocabulary is exclusively of physical events. Any reference to a conscious experience of seeing would be irrelevant to this physiological account.
Described above is a very simplified version of an amazingly complicated set of processes. This operation, continuous all the time our eyes are open, is a natural phenomenon of staggering wonder. But where is seeing itself in the midst of all this chemical complexity? The conscious experience is instantaneous. I shut my eyes, then open them. I don’t have to wait for the image to be imprinted and chemically transmitted. It happens without any delay; so fast is the processing from cornea to cortex. But where, I repeat, is seeing in all this, the experience of seeing itself? What is it? How does it come about? What I mean by seeing is the felt phenomenon of seeing the tree. It seems that the explanation pursued above tells me a great deal about the biology of visual mechanisms associated in some as yet unexplained way with seeing but absolutely nothing about the experience of seeing itself, of what it is like to see. It does not even begin to explain what seeing is like as a conscious event. It is this, the conscious experience, which the biological textbooks understandably ignore. And it is precisely at this point that we encounter the essence of the mind-body problem in this different context and one reason why it has proved so intractable to rational enquiry. One can envisage that as time passes and research is extended, biologists will be able to provide an ever more detailed and far-reaching analysis of the physical processes of vision but no matter to what level of complexity this description is extended, by its very nature it will not include the conscious experience of seeing. Seeing, that is, the conscious experience, is not an object of scientific investigation.
It is very tempting to imagine that at the end of the biological process there sits an observer watching a screen on which is displayed an image of the external world in the range of the eyes, an inner person on a smaller scale, a homunculus to whom the whole visual show of life is presented. Unfortunately, there is no evidence of any such internal observer. Even if there were such an entity, far from resolving the problem it would simply repeat it. For, if a miniature version of me were watching a miniaturised screen, an account would need to be given of the physical processes by which reflected light from the screen impinged on the eye and triggered a series of responses in the homunculus brain with the result that an experience of seeing took place. Clearly, this is the same problem presented earlier but on a smaller scale. The unrewarding prospect of an infinite series looms at this point in the argument as within the brain of the homunculus lives a micro-homunculus and within the brain of that - and so on. This is no answer to the problem of visual awareness, which remains daunting. How can it be that a continuous visual world evident in conscious experience arises out of the electrochemical circuitry of ‘a slurry of tissue with the consistency of raw egg’ ? It defies rational explanation and yet it happens.
This brief study of visual processes has raised two points:
a) that the conscious state of seeing occurs after a change in the physical state of the brain caused by light rays impacting on the brain
b) that the conscious state of seeing is outside the chain of physical events taking place in the brain’s visual system and is inexplicable in terms of those events.
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