c CUNYB/Clarke December, :
The ScientificEssaysand theDiscourse on Method This claim requires an account of how the nerves work and how they
can transmit information from the external senses to the brain. Descartes
adopts the common opinion of anatomists, that a cross-section of nerves
shows three distinct parts: an outer membrane, an inner filament, and a
very subtle matter (called animal spirits) that lubricates the gap between
the outer and inner layers and thereby allows the inner tube to move
smoothly within the outer membrane. If one assumes that information is
transmitted from the external senses to the brain by the motion of the
inner filament, one can exploit the analogy of the blind man to show that
the mind can acquire reliable information without having an image of
perceived objects. At this point Descartes repeats an argument fromThe
World, and, since that book remained unpublished, he could present it to
the public as if for the first time. ‘There are many other things apart from
images which can stimulate our thought such as, for example, signs and
words, which do not in any way resemble the things they signify’ (vi.).
The argument is that, if words can stimulate the mind to think of specific
things without resembling them – the word ‘horse’ does not in any way
look like a horse! – then why would it not be equally possible for the mind
to think of a horse as a result of patterns of information from the optical
nerves, without this information resembling a horse?
The optical part of this discussion is presented in the fifth discourse,
in which Descartes accepts that an optical image resembling an object of
perception is formed on the back of the eye. However, the novelty of his
contribution is in the sixth discourse, in which he constructs a theory about
how the information presented in this optical form can be transmitted to
the centre of the brain. He repeats the general principle that ‘our soul is of
such a nature that the force of the movements’ of the optical nerves ‘makes
it have the sensation of light’, although ‘there is no need, in this whole
process, for any resemblance between the ideas that the soul conceives and
the movements that cause those ideas’ (vi.,). The Cartesian theory
is that the brain receives information from many sources apart from the
optical image on the retina – for example, from the muscles that move the
eyes, from muscles that adjust the focal length of the ocular lenses, from
the degree of brightness of images received, and from changes in the size
of the pupil.
The most explicit example of ways in which information is made avail-
able to the brain is developed by analogy with a blind man who estimates
the distance of some object by using two sticks. The blind man knows that