Tuesday, September 1, 2015

2 Self-knowledge and neurophysiology – a reply to David Parker

David asked: ‘Why do you say that consciousness has to be non-corporeal?’

In my preceding post I based my answer on Aristotle’s view that two bodies cannot be at one and the same place: From the point of view of anatomy and neurophysiology our head is packed with brain; our consciousness presents us with space packed with physical structures, which are organized completely differently in space and time from the way the brain with its neural networks is organized. All we are conscious of is mediated to our consciousness by the brain activities in our head; therefore, our consciousness too must be located in the head. Since two bodies cannot be at one and the same place, consciousness must be non-corporeal.

In the De Anima Aristotle notes that we discriminate between sensory perceptions provided by different sensory organs – when we consider a sugar cube, we are aware that its being sweet is different from its being white – and that this discrimination is sensory and must therefore be perceived by ‘the ultimate sense organ’ (to eschaton aisthêtêrion, 426b16). He argues that these differences must be clear (dêla einai) to something that is one (heni tini) and undivided (achȏriston) and in undivided time (kai en achȏristȏi chronȏi, 426b18-29); this is why it cannot be corporeal (426b15); in the De Anima Aristotle reemphasizes the principle that two bodies cannot be both (hama) in ‘one and the same’ (en tȏi autȏi, 418b17).

Aristotle’s idea that there must be ‘the ultimate sense organ’, which integrates into one perceptions that are provided by different sense organs, has acquired new urgency in neurophysiology, which can trace sensory stimuli from different sensory organs to their corresponding brain centres, but can’t find any ‘brain centre’, where the sensory information is integrated into the ‘sensory scene’ of our consciousness.

Aristotle argues that if two such perceptions as ‘white’ and ‘sweet’ were to be perceived as different by two different sensory agencies, ‘it would be the same as if their mutual difference were to be made clear by my perceiving the one and you the other; but it is the one that must say that they are different (dei de to hen legein hoti heteron), for the sweet is different from the white; it is therefore the same one that says this (legei ara to auto); so that as it says this, so it thinks and perceives it’ (426b19-22).

As I look in front of me, my visual field is divided into objects, each of which is perceived as a differentiated unity: the computer screen displaying the text I am writing, each sentence differentiated into words, words into letters … Each of these differentiated unities is conveyed to my brain by thousands of neurons, each of which is an individual agent, functionally connected with other nerves by the actions of transmitters on synapses. How can these individual neurons form the differentiated oneness of my conscious experience of the text I am writing? To use Aristotle’s example, it is like bringing together thousands of men, each of them seeing and reading a different letter, and expecting them to perceive and understand jointly what I am writing.

The urgency of this problem concerning neurophysiology is well illustrated by David in his reply to my ’Notes on the relevance of neurophysiology to self-knowledge’ posted on July 30: ‘Much of the neurobiological evidence focuses on the analysis of visual pathways. Cognition (and thus mind, if you consider this a fair link?) arises from anatomical and functional aspects: various people (Hubel and Wiesel, Semir Zeki, David van Essen) have shown how simple features such as lines, colours etc.. in the visual cortex arise from lower level processes in the cells and synapses in the retina (already at the retina there is a complex network with several uncertainties of how it works), and how these are used to form perceptions ultimately of the form and position/movement of objects in a visual scene. There is the idea of a “grandmother” cell (more recently the “Jennifer Aniston” cell), a hypothetical cell that is the final point  in the visual cortex processing hierarchy where all the features of dots of light, lines, edges, colour, form, movement, maybe also memory and emotional components, are integrated to give the conscious perception of a face. This is unlikely to be a single cell; we lose many neurons every day and if one cell was responsible we would randomly lose whole percepts. But it could be a network of cells, and when all the features of a grandmothers face, the memory, and the associated feelings are brought together the circuit is active and this gives rise to the thought or actual perception of that face.’

It may be asked, why Aristotle saw the necessity of perceiving sensory differences by something that is one and undivided, therefore non-corporeal, only when he was considering the problem of ‘the ultimate sensorium’. The reply is simple. When Aristotle considered the functions of the specific sense organs, he viewed them in relation to things perceived: ‘The sense perception (hê aisthêsis) is that which receives the forms of perceived objects without their matter, just as a piece of wax receives the sign of a signet-ring without the iron or gold (De Anima 424a17-20).’ Aristotle believed that by our senses we perceive perceptible things simply as they are. Neurophysiology, which compels us to view the problem of sensory perception face to face with our brain and its neural networks, makes Aristotle’s view that ‘sensory differences must be perceived by something that is one and undivided, therefore non-corporeal’ relevant to every aspect of our sense perception.

Let me illustrate this point with one more quote from David’s reply posted on July 30: ‘Problem of qualia, individual subjective component of perception. Schrodinger said, “The sensation of color cannot be accounted for by the physicist's objective picture of light-waves. Could the physiologist account for it, if he had fuller knowledge than he has of the processes in the retina and the nervous processes set up by them in the optical nerve bundles and in the brain? I do not think so” … That I have a subjective impression of a “redness” of an object that is specific to me does not seem to negate physicalism, that this impression is formed from interactions between nerve cells, just as a perception of a straight line is formed that way.’

But is it true that a perception of a straight line is formed from interaction between nerve cells? The only thing neurophysiology can tell us, as I understand it, is that in the visual cortex we can find units that are able to respond to a specific type of stimulus, such as a straight line, but this does not mean that these units form a perception of a straight line. If one day neurophysiology finds units in the visual cortex that respond to different colours, this does not mean that it will have discovered how the perception of colours is being formed.

Aristotle on his view of sensory perception had a simple explanation of qualia. In order to be able to see white and black, our sensory organ of vision must be neither black nor white actually (mêdeteron autȏn einai energeiai); but potentially it must be both (dunamei d’ amphȏ); it becomes actually black when it perceives black, white when it perceives white (De Anima 424a6-9)

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