Some Notes on Reversible Perspective: Part I, Sensation as Sign- continued
© Peter Macapia 1995
In his writings on physiological optics Hermann von Helmholtz made extensive comments on Wheatstone, upon whose work he was to build his theories of unconscious inference and sensations as signs. What Helmholtz added was a more refined explanation of how perceptions manifest content about the world; that one could derive meaning from sensations in the absence of an external signifying structure; that understanding could emerge from the labryinth of the nervous system. He did this by showing how the formation of our perceptions is similar to the way in which we learn language.
For Helmholtz, Wheatstone's stereoscope was an irrefutable demonstration of unconscious inference and that sensations were mental signs. As he emphasized in his Popular Lectures, it was the clearest proof that seeing with two eyes, and the difference of the pictures presented by each, constitute the most important cause of our perception of a third dimension in the field of vision.  But the Necker cube and other simple geometrical objects were also crucial in this respect. Geometrical line drawings of solids without shading or color, rather than photographic images, best illustrated the fact that spatial perception was not conveyed by anything other than the mental synthesis of two images. 
In his study of stereoscopic representations of crystals, Helmholtz demonstrated that the nativist theory that two retinal images fuse physlogically and produce a single picture was fundamentally wrong.  Stereoscopic drawings of crystalline forms, which on one plate were white and on the other black, produced an impression of a solid crystal of shining graphite. The fact that the combination of black and white produced a lustrous image rather than a grey one was evidence that no physiological fusion of the images occurred before they reached the brain, as well as evidence that the brain actually received two images. The perception of space therefore, "was not produced by any anatomical mechanism of sensations, but by a mental act." 
But the nativist theory remained in some respects unchallenged, for if the fact that, say, three distinct points in one's visual field, i.e., on the retina, was not cause for assuming that they originated from three different locations in space, then what produced the perception of space? The basic nativist conception, such as Brewster's or Hering's, held that the retina in a sense extended into and therefore connected with space. The retina reacted as an intelligent organ, according to Brewster, receiving impressions which carry with them this quality of extension in space, adding that impressions derived from external objects are transmitted of themselves to corresponding local positions in the image produced in the sensitive organ. The perception of space was thus an inherent property of vision.
Following Wheatstone, Helmholtz's refutation of the nativist theory was tied to the criticism of what were held to be external and independent objects. A perfect illusion, such as provided by the stereoscope, confirmed his theory that sensations were really signs and therefore susceptible to confusions in meaning. If, for Brewster, sensations were the very facts of the existence of objects, that perceptions arose in their very presence, then the Wheatstone stereoscope, according to Helmholtz, suggested quite the opposite: the stereoscope confronted vision with its own corporeal muteness: the eye was only a receptacle of sensation, not an intelligent organ. Our sensations do not, Helmholtz asserted, give us anything more than signs for external objects and movements. And we learn how to interpret these signs by means of experience and practice in order to arrive at knowledge of the external world. 
Let us consider now Helmholtz's empiricist conception of objects of perception and sensation. The properties of so-called external objects, therefore, may only be regarded as effects. They are not peculiar to the individual object itself but invariably imply some relation to a second object -- our organs of sense. 
Our apperceptions and ideas, therefore, are effects wrought on our nervous system and our consciousness by the objects that are thus apprehended and perceived.  That there are external objects is a deduction from our experience. When sensations do become perceptions, it is only through interpretation, which in turn is formed through an association of ideas that, for the most part, are produced by "unconscious and involuntary activity of the memory."  Clearly, knowledge of objects is mediated. Yet we do directly apprehend sensations, the changed condition of the nervous fibers which we call the state of excitation, something unaffected by the will, and physiologically explicable. 
But the fact that they are interpreted and associated unconsciously proves that they function like signs. And this requires a psychological explanation. For regarded as simply symbols of relations in the external world they are denied every kind of similarity or equivalence to the things they denote. 
Now according to the nativist point of view, optical illusions presented the strongest case against Helmholtz's empiricism. For if the meanings of sensations are learned from experience, then they always ought to agree with experience. Optical illusions were precisely the case of one's confusing the meanings of one's sensations. In responding, Helmholtz explained optical illusions in terms of unconscious inference. We transfer notions of external objects, which would be correct under normal circumstances to cases in which the unusual circumstances have altered the retinal pictures.  These transfers are merely unconscious inferences that turn out to be false, false inductions, or unconscious judgments.
But we must note here how Helmholtz supplements the psychological description of sensation by stressing the different kinds of knowledge possessed by a subject; publicly informed knowledge of facts and privately formed knowledge of sensations.  Unconscious judgements do not occupy us in the same way as conscious thoughts, in that we do not check our observations to see whether they are justified. This problematizes the fact that a judgment just is a logical conclusion, the result and final stage of a conscious operation of the mind. Helmholtz's response to this question was that the judgements which play so great a part in the perception we derive from our senses cannot be expressed in the ordinary form of logically analyzed conclusions. And so it is necessary, he added, to deviate somewhat from the beaten paths of psychological analysis in order to convince ourselves that we really have here the same kind of mental operation as that involved in conclusions usually recognized as such. Helmholtz elaborates:
[there] appears to me to be in reality only a superficial difference between conclusions of logicians and those of inductive conclusions of which we recognize the result in the conceptions we gain of the outer world through our sensations. The difference chiefly depends upon the former conclusions being capable of expression in words, while the latter are not; because instead of words, they only deal with sensation and the memory of sensations. Indeed, it is just the impossibility of describing sensations, whether actual or remembered, in words, which makes it so difficult to discuss this department of psychology at all.
Helmholtz remarked that this kind of knowledge, Kennen, which cannot be expressed in words, is as certain and precise as Wissen, which can be expressed in words. But what he based this distinction on, as well as a simultaneous structural similarity, was a model of language. Helmholtz's theory, then, illustrates the structural similarity between the apprehension of words and sense-data. Now this connection between Names and Objects, he stated, which demonstrably must be learned, becomes just as firm and indestructible as that between sensations and the objects which produce them. 
And yet Kennen involves recognition, familiarity with how to do something. It is not publicly taught. It is not acquired through language, but rather learned privately through experience and experiment. Even more important, sense-data have a logical grammatical structure: it is clearly possible, by using these sensible images of memory instead of words, to produce the same kinds of combination which, when expressed in words, would be called a proposition or a conclusion.  The only confirmation of this form of knowledge is introspection, but it cannot be proven because it does not fall within the bounds of scientific discourse.
Helmholtz's key assertion is that unconscious and conscious inferences are equally accurate because they are based on similar structures of knowledge; the elementary bits of language and experience, of words and sense-data. But this is something that is imposed on us by the nature of the world. For sensations as signs reflect the order of the world by virtue of how we learn them in our subjective experience. The ability to view the world with understanding is made possible by the processing of sensate experience into representations that have meaning; that the abstract relays between stimulus and sensation must have the organization of a representation that corresponds to reality and is usable by the subject; and that sensations have "meaning" in the same way that words in languages do (language and sensations as signs reflect the atomic structure of reality) even though the structure of language may derive from the more primitive structure of sensations as signs. 
What the model of language allowed Helmholtz to avoid was the problematic abyss between knowledge and the external world.  There is a "striking analogy" between the process of associating sensate experience and another "system of signs" which is not natural but arbitrary, "the words of our mother tongue."  In learning how to speak, a child must first "guess that the sounds it hears are intended to be signs at all; next, the meaning of each separate sound must be found out, by the same kind of induction as the meaning of the sensations of sight or touch. . ." 
Now this connection between Names and Objects, which demonstrably must be learned, becomes just as firm and indestructible as that between Sensations and the Objects which produce them. "The elementary signs of language are only twenty-six letters, and yet what wonderfully varied meanings can we express and communicate by their combination! Consider, in comparison with this, the enormous number of elementary signs with which the machinery of sight is provided . . . [I]t is possible to construct an immeasurably greater number of combinations here than with the few letters which build up our words . . . [With] the extremely rapid changes of which the images of sight are capable . . . our senses speak to us in language which can express far more delicate distinctions and richer varieties than can be conveyed by words." 
As a conclusion, one could say that the model of subjectivity in Helmholtz's theory of perception implied a subject who stood at the center of language and sensations, connecting them to the world through ostensive definitions based on private experience. And in this way one could also say that Helmholtz's, or for that matter, Wheatstone's model of subjectivity begins to appear less and less distinct from Brewster's. A subjectivity, that is, beyond language and things. For, in view of Part II of this essay, we shall see how Helmholtz's theory that words are to language what sensations are to thought involves conceptual confusions over the identity of sensations: a confusion involving the means for describing sensations, methods of deriving them (introspection), and the general state of their objecthood.  A series of problems that throws into question not just the nature of sensation, but of experience and subjectivity.
As I will discuss in the second part of this essay, Ludwig Wittgenstein had developed an account of subjectivity similar to Helmholtz in his Tractatus Logico Philosophicus. Indeed he used a version of the Necker cube as an illustration. Yet Wittgenstein's middle and later work were intended to disqualify his earlier account of private ostensive definition. His criticisms were formulated out of a series of studies of cases of reversible perspective.
I would like to thank Yve-Alain Bois who first brought the Necker cube to my attention as a potential research project for his seminar on axonometry.
part two to appear in basilisk v1.2
© Peter Macapia 1995