Descartes on pain
Rene Descartes Sixth Objections and Replies
Reply (9) To get a clear view of what kind of certainty attaches to the senses, we must distinguish three levels of sensory response.
- (a) There is the immediate stimulation of the bodily organs by external objects; this can only be the motion of the particles of the organs, together with any change of shape and position resulting from this motion.
- (b) There are the immediate effects produced in the mind as a result of its being united with a bodily organ that is affected thus and so. Examples are the perceptions of pain, pleasure, thirst, hunger, colours, sound, taste, smell, heat, cold and the like, which arise from the union—the intermingling, as it were—of the mind with the body, as I explained in the sixth Meditation.
L (c) Then there are the judgments about things outside us that we have been accustomed to make from our earliest years—judgments that are triggered by the movements of these bodily organs.
When I see a stick, what happens?
- A wrong answer: certain 'intentional images' fly off the stick towards the eye.
- The right answer: rays of light are reflected off the stick and set up certain movements in the optic nerve and, via the optic nerve, in the brain, as I have explained at some length in my Optics.
This movement in the brain is the first level of sensory response; we have it and so do the brutes. It leads to the second level, which takes in the bare perception of the colour and light reflected from the stick; it occurs because the mind is so intimately conjoined with the body that it is affected by the movements that occur in the body. If we want a clean line between the sensory faculty and the intellect, those two levels are all we should attribute to the former. But now take a case where this happens: I am affected at the second level by a sensation of colour. This leads me to judge that a certain physical stick is coloured. Also, on the basis of the layout of the colour and its boundaries, together with its position in relation to the parts of my brain, I rationally calculate the size, shape and distance of the stick.
Such reasoning is commonly assigned to the senses (which is why I called it the third level of 'sensory' response), but clearly it depends solely on the intellect.
I demonstrated in the Optics how size, distance and shape can be perceived by reasoning alone, which works out any one of those features from the other two. 'What we have here is a difference not of process but only of circumstances. When we have some new kind of observation, and arrive at our first judgment about it, we think of this judgment as the work of the intellect. When we have some sensory input of a familiar kind, and arrive at the old familiar judgment about it, we think of this judgment as coming from the senses. Yet the process of judgment-formation is exactly the same in the two cases, so why do we have a feeling that they differ? It is because in the latter case we work our way to the judgment at great speed because we have done this so often; or rather we remember our past judgments about similar objects •and copy them in the present case, thus not having to 'work our way' to the judgment at all. This shows that when we say 'The •intellect is much more reliable than the •senses', we mean merely that •the judgments we make as adults on the basis of various new observations are more reliable than the judgments we unreflectively arrived at in our early childhood; and this is undoubtedly true. The first and second levels of sensory response don't come into this, because there can't be anything false in them. So when people say that a stick in water 'appears bent because of refraction', they are not attributing falsehood to the second level of sensory response, but saying in effect that the stick appears to us in a way that would lead a child—or would lead us if we weren't careful—to judge that it was bent.
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