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In Context #15 (Spring, 2006, pp. 3-6); copyright 2006 by The Nature Institute

The Light of Sense Experience
Georg Maier

This article consists of excerpts from two books. The immediately following text is from Optik der Bilder (1986) by physicist Georg Maier (excerpt translated by Henrike Holdrege and Stephen L. Talbott). The second half of the article is taken from a new book entitled Being on Earth: Practice In Tending the Appearances, co-authored by Maier along with the late Stephen Edelglass (also a physicist) and the late Ronald Brady (a philosopher). See below for further information.

The development of natural science over these last centuries has brought an increasing distrust of sense perception. This is remarkable, since during this same time nature observation and experiment were exalted as the sole sources of experience. The apparent contradiction dissolves when we recognize what sort of reality the leading thinkers were willing to acknowledge. The program laid down by such pioneers of natural science as Descartes and Locke was guided by a particular criterion of reality: as essential properties of things they accepted only "primary" qualities - that is, position (locus), movement, spatial form - qualities readily yielding to mathematical treatment.

By contrast, the "secondary," inessential qualities included sound, taste, smell, and sensations of warmth, but also the qualities of sight such as brightness and darkness. Because of their "merely apparent" character, these qualities were understood as pointing toward underlying structures of the primary kind. This requires us to develop mental images relating the fleeting sense qualities to bodily (physical) processes.

The history of modern physics is closely connected to changing viewpoints about the nature of light. The mood of those earlier times can be felt in the way scientists struggled to interpret the "spreading out of light in space" in terms derived from the experience of one's own physical body. To illustrate this, here is a quotation from Descartes' treatise on Optics (1985, p. 153):

No doubt you have had the experience of walking at night over rough ground without a light, and finding it necessary to use a stick in order to guide yourself. You may then have been able to notice that by means of this stick you could feel the various objects situated around you, and that you could even tell whether they were trees or stones or sand or water or grass or mud or any other such thing. It is true that this kind of sensation is somewhat confused and obscure in those who do not have long practice with it. But consider it in those born blind, who have made use of it all their lives: with them, you will find, it is so perfect and so exact that one might almost say that they see with their hands, or that their stick is the organ of some sixth sense given to them in place of sight. In order to draw a comparison from this, I would have you consider the light in bodies we call "luminous" to be nothing other than a certain movement, or very rapid and lively action, which passes to our eyes through the medium of the air and other transparent bodies, just as the movement or resistance of the bodies encountered by a blind man passes to his hand by means of his stick. In the first place this will prevent you from finding it strange that this light can extend its rays instantaneously from the sun to us. For you know that the action by which we move one end of a stick must pass instantaneously to the other end, and that the action of light would have to pass from the heavens to the earth in the same way, even though the distance in this case is much greater than that between the ends of a stick. Nor will you find it strange that by means of this action we can see all sorts of colors. You may perhaps even be prepared to believe that in the bodies we call "colored" the colors are nothing other than the various ways in which the bodies receive light and reflect it against our eyes. You have only to consider that the differences a blind man notes between trees, rocks, water and similar things by means of his stick do not seem any less to him than the differences between red, yellow, green and all the other colors seem to us. And yet in all those bodies the differences are nothing other than the various ways of moving the stick or of resisting its movements. Hence you will have reason to conclude that there is no need to suppose that something material passes from objects to our eyes to make us see colors and light, or even that there is something in the objects which resembles the ideas or sensations that we have of them. In just the same way, when a blind man feels bodies, nothing has to issue from the bodies and pass along his stick to his hand; and the resistance or movement of the bodies, which is the sole cause of the sensations he has of them, is nothing like the ideas he forms of them. (Descartes 1985)

By comparing the spreading-out of light to a stick transmitting shocks from one body to another, Descartes encouraged a line of thinking that would later be elaborated into the wave theory of light: light is the condition of movement of a medium. This was hard to imagine because light also spreads out in a vacuum - that is, in the void. So the void, or vacuum, had to be understood as the light's transparent medium.

In the history of physics this point of view contradicted another one, which held that light consists of minute balls that can fly through air, glass, and, best of all, empty space. Isaac Newton took this view. For him, different colors were particles of different kinds. In the nineteenth century the wave concept triumphed over the particle concept. In particular, Augustin Fresnel discovered new phenomena, which clearly had to be explained as the mutual canceling and amplifying of light waves. Moreover, electromagnetic (radio) waves were produced and showed the same phenomena. And so light came to be understood as electromagnetic oscillation, and the different colors as oscillations of different frequencies or (seen spatially) different wavelengths.

In the twentieth century, however, the concept of particles revived. In near darkness one could observe effects of light concentrated in single events - light quanta. Max Planck showed that the radiation of hot bodies could also be understood on the assumption that light is emitted in the form of quanta - quanta that occur on a certain random basis. Every twist in this barely sketched historical investigation was understood as a step in the discovery of light's true nature. Followers of the different theories fought with each other, because light must consist either of waves or particles.

One might now say, "Science has proven the particles to be 'light quanta'; therefore they travel around." However, anyone who studies the history of models in physics knows that the particles do not manifest themselves "along the way," but only if you disrupt the optical context you are investigating. If you try to determine the "route" of the particles by introducing a particle collector between a light source and an object it illuminates, you immediately create a new and different situation. Therefore the physicist does not say any longer, "Light consists of particles transported through space." Strictly speaking, nothing matter-like streams through space. Instead we find various contexts involving spatially separated loci, wherein causes at the one locus call for effects at the other locus....

Despite the twentieth-century development of quantum mechanics in physics, the mental image of a particle-stream remains deeply rooted today.... People say, "When a lamp illuminates a surface that is separated in space from the lamp, there must be a physical process in the space between - a process mediating between the cause (shining lamp) and effect (illumination of the surface)." And they are convinced that only through mental images of such processes can we understand the lawfulness of illumination and its causes.

So the imagined processes are placed exactly where there is no visible phenomenon. The lamp is visible, and the surface illuminated by it is visible. Can we understand the relation between the two by renouncing mental images of the mediating physical processes we have invented?

From Being on Earth: Practice In Tending the Appearances. The passage reproduced here is from a chapter written by Maier and entitled "Sense Perception as Individual Experience: Pursuing George Berkeley's Thoughts on Vision."

The Law of Illumination

Although the moon is brightly illuminated by the sunshine, the sun's "light" that supposedly flows through space to be reflected by the moon is not itself a visible phenomenon! This means our Berkeleyan visual standpoint (Berkeley 1953) does not allow us to invoke such flowing light in our explanations. What we definitely do know is that sources of illumination are especially bright "things of sight," as Berkeley would say. And these must be visible from any surface they are illuminating. Putting this in other words, we may formulate the following principle: objects light up according to their visible surroundings. On these terms alone - and without reference to flowing light - we will be able to explain the diminishing brightness of illumination at increasing distance from a lamp.

In the usual explanation, illumination is understood to be caused by rays emanating from a source (often idealized as a point source) and spreading outward. The nearer an object is to the source, the more rays it intercepts and therefore the brighter it appears. (Illustration from Maier 1986.)

Illumination is usually explained as follows. Imagine light to be steadily issuing from a lamp in all directions. We assume the surrounding space to be perfectly clear, so no light is lost as it spreads into space. But the light must expand, so that its power to illuminate is distributed over surfaces of greater extent at greater distances. Take the lamp to be located at the center of a sphere, with the flow of light distributing itself evenly over the surface of the sphere. The area of this surface grows in proportion to the square of the sphere's radius. Thus, as distance from the lamp increases, the illuminating effect of the lamp diminishes, corresponding to the reciprocal of the square of the distance.

This argument depends on our imagining ourselves to be observing light as it crosses space in front of us, as if its movement could be seen from the side. We are all used to imagining this. But if we remind ourselves of the appearance of the moon at night - where we do not see sunlight streaming toward it - we will have to admit that this habit is not supported by experience. Berkeley did not like it. But it has hardly been noticed that his approach - which is meant to rest on sense experience - leads to an alternative train of thought that is just as useful in its result. Doing without the imagined viewer observing a stream of light from the side, we can deal with the problem of illumination this way:

We take lamps to be "objects of sight." That is, they gain in visible size as we move toward them and diminish in visible size as we move away from them. This is the effect of perspective. And as we will see, this change in visible size is sufficient to give us the law we are seeking. Let us again assume that the atmosphere is perfectly clear. Then we can convince ourselves that the seen brightness of a lamp does not change with distance. That is: if we view two identical frosted lamps with the second one at a greater distance than the first, and if we allow the first one to overlap our view of the second, then we will readily observe that they appear equally bright. The two bright discs will merge.

What changes with distance is not the brightness, but the visible area of the lamp exhibiting this brightness. The visible area alone determines the illuminating effect at a given distance from the lamp. (Of course, inside science one does not say "visible area"; one speaks of the "solid angle" subtended by a luminous surface.) According to the laws of perspective, the visible area of a lamp will diminish according to the inverse square of its distance from the observer. So we have gained the same result we did above - but by speaking of the visible area of the light source rather than invisible rays moving through space. Outside the immediate vicinity of the lamp we get exactly the same simple law as above. And since we have given up the usual idealization which treats the lamp as a point source, our formulation of the law now deals with the problem of illumination in the immediate vicinity of the lamp - a problem that the point-source idealization cannot handle, namely, the fact that the illumination remains proportional to visible size. (The hypothetical "point source" from which the light is supposed to stream out into space is not given in reality - it would be physically impossible and, moreover, the calculated illuminating effect of such a point at close range would not be what we actually observe. On the other hand, the lamp that is more realistically taken to be of the nature of "things of sight" just grows in solid angle the nearer you approach it, consistent with the observed law of illumination.)

The outdoor lamp next to the house number "7" increases in apparent size as one approaches the house, and so does the illuminating effect of the lamp. (Illustration from Maier 1986.)

Note that by relating the apparent size of the lamp, its visual quantity, to its effect as an illuminant, we no longer need to assume that light transports itself through space, at least in the context of problems of illumination. But even in a much wider context modern physics tends to give up the notion of light traveling through space in the way bodies do. For example, we learn from principles of optical imaging that the precision of the image deteriorates as the line of sight (that is, the presumed path of "flowing" light) from object to image is defined more exactly. This can easily be demonstrated. Reduce the aperture of the eye's lens by looking through a tiny hole pricked into a piece of paper. In this way you define the sight path (the imagined "path of light") with greater precision. But the result is a blurring of your sight. The image deteriorates while your knowledge of the path between it and your eye becomes more accurate. On the other hand, the big telescopes used in astronomy, with their huge openings pointed into the sky, "see" an ever so finely structured scene. This reciprocal relation between precision of the line of sight and quality of the resulting image suggests that the supposedly intrinsic, ray-like character of light is really an artifact of the mind, an artifact that has been handed down from generation to generation.

Georg Maier's Optik der Bilder is currently being translated into English for publication, perhaps in two years, by Adonis Press. The forthcoming Maier/Edelglass/Brady book, Being on Earth: Practice In Tending the Appearances, is now being readied for publication on The Nature Institute's website ( The book is an attempt to show (as stated in its introduction) that "a truly phenomena-based science has radical implications for understanding sense experience and the world of phenomena."

After taking his Ph.D. in physics in 1960, Georg Maier spent about seven years doing nuclear reactor-based research, particularly in the field of neutron optics. From 1969 to 1998 he worked at the Research Institute of the Goetheanum in Dornach, Switzerland, pursuing investigations in many fields of physics and publishing numerous papers. Now retired, he continues his researches in Dornach, where he lives.

Georg's life-long physical investigations have been gaining a living presence at The Nature Institute, particularly through Henrike's work in optics. For example, her seminar on "Seeing with Fresh Eyes: Phenomenological Exploration of the Visual World" (part of this spring's Goethean Science course) is substantially founded on Georg's work.


Berkeley, George (1953). Berkeley: Philosophical Writings, edited by T. E. Jessop. Austin TX: University of Texas Press. Berkeley's "Essay Towards a New Theory of Vision," originally written in 1709, is contained in pp. 1-32 of this book.

Descartes, René (1985). The Philosophical Writings of Descartes, vol. 1, translated by John Cottingham, Robert Stoothoff, and Dugald Murdoch. Cambridge: Cambridge University Press. The treatise on "Optics" (pp. 152-75 of this volume) was originally published in 1637.

Maier, Georg (1986). Optik der Bilder. Verlag der Kooperative Dürnau. Last reprinted in 2003.

Maier, Georg, Ronald Brady, and Stephen Edelglass (forthcoming). Being on Earth: Practice In Tending the Appearances. This book will soon appear on The Nature Institute's web site.

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