1.         To a large extent, my friend’s impressions of the piece of chalk depend on her own perspective rather than on the chalk itself. She says that the chalk is purely white, but when she actually looks at the chalk she sees a variety of shades of shadow. When I look at the chalk, I see a different set of shadows, and if we were to go somewhere with colored light, we would see another entirely different set of colors. Clearly, the chalk’s color is dependent on the environment it is in and the location of the viewer. To make matters worse, color itself is the brain’s way of interpreting certain wavelengths of light, rather than an inherent property of the chalk itself. If humans had happened to evolve so that “visible light” was at some other place on the electromagnetic spectrum, my friend and I wouldn’t see the chalk as white but as some other entirely unimaginable color.1

According to Russell, similar problems occur with my friends other claims. To human-sized beings, the chalk appears smooth, but if my friend were about 0.26 mm tall, the chalk would appear to be a lumpy collection of intricately designed spheres. My friend and I, looking at the chalk from different vantage points, also see slightly different shapes that aren’t quite cylinders, although we are accustomed to ignoring this difference in perspective. It goes like this for every possible claim about the chalk, suggesting that my friend is in error when she says that there can be no doubt about her observations. Russell suggests a more valid claim: There is no doubt that my friend really has the sensations that she thinks she does. She experiences a valid set of sensations, even if it not the only possible set of sensations the chalk could create and does not represent the most fundamental level of the chalk’s existence.

To me, Russell’s arguments about color and sound are far more intuitive than his arguments about shape and texture. Color and sound, after all, are more products of human eyes, ears, and brains than anything else. Without eyes to see, there would still be light of various wavelengths bouncing around the world getting absorbed and reflected. However, the term “color” would have no meaning, because color is a type of human qualia rather than a separate characteristic of objects. Likewise, without ears, there would still be vibrations in the air, but they would just be vibrations; the sensation of sound as we typically think of it (in music, speech, etc.) would disappear. As an illustration of just how viewer/hearer-dependent these two qualities are, I once heard of a color-blind man who “saw” color with sound. He wore a sensor that played a slightly different tone for each color. For example, when the man looked at a red and blue striped shirt, the sensor played a combination of the notes for red and blue. For this man, the experience of color was completely different from most other people’s experience.

Shape and texture do not seem to me to be this subjective. I do not create the chalk’s cylindrical shape just by looking at it; if no living thing ever existed to look at the chalk, it would still be cylindrical. Because of this, I’m not sure I understand the difference between the sense-data and the chalk itself in this case.2 With texture, the sensation of skin-on-smooth-chalk may again be a product of the skin as much as the chalk, but the texture itself (those qualities that produce the “smooth” sensation) exists more independently than the sensation. (In the same way, the chalk would still reflect the same wavelengths of light even without sight to make the sensation of color relevant.)3


2.         The sense-data are qualities such as the rose’s red color, its fragrance, or the sharp stab of its thorns. The sensation is my awareness of these qualities, for example my experience of the scent of the rose. The sense-data are a degree removed from the rose itself. The rose causes me to experience certain sensations, but I cannot understand the true “essence of rose;” the best I can do is to experience the sense-data.

For the question of whether things exist when no one is looking at them, I like an explanation given by Hans Reichenbach in The Rise of Scientific Philosophy. His idea is that we cannot know for sure that objects continue to exist when we don’t see them, but if we assume they do, unseen objects obey all the same laws that observed objects do. This satisfies the goal of creating an internally consistent set of beliefs about the world. However, it is only one of a variety of potential linguistic conventions. We could theoretically come up with a set of conventions in which we assumed that unseen objects do not exist, but we would also have to create two sets of physical laws, one for seen objects and one for unseen objects. Reichenbach uses the example of a house casting a shadow. Viewers look at the house and understand that it casts the shadow because of the everyday rules of optics. The viewers then turn away from the house, but they can still see the shadow. If they adopt the convention that the house still exists, they can accept that the shadow is still cast for the usual optical reasons. If they adopt the convention that the house doesn’t exist, they must adopt a second optical system in which nonexistent objects can cast shadows.

For questions like this, simpler answers are generally more convincing. Assuming the continued existence of unseen objects is a simpler--and therefore superior--convention to adopt.



1. Whether or not there exist such entities as “unimaginable colors” is an interesting question.


2. It might be helpful to consider the challenge of computer scientists working in artificial intelligence.  How do you write a program that receives a variety of 2D pictures of an object (perhaps as it is rotated) and then converts those numerous patterns of lines and shadows into the correct 3D model of the object?  Evolution has solved that problem so well for us that it’s hard to realize the actual inherent difficulty.


3. This may be a matter of definition.  If you define “texture” as the three-dimensional pattern of matter composing the object, then most (except for the extremely skeptical) would say that pattern exists independent of an observer.   But if you define “texture” as that which is felt, it becomes much more similar to color.  Likewise, a can of soup is a cylinder when held in your hand, a point when viewed from across a football field, and a plane for the microscopic bacterium living on its surface.