What are shadows? Can we literally see them? Can we only see them, or can we also feel them? Do they have colours? If so, can they have chromatic colours, or are they only ever black or grey? How do shadows differ from reflections and silhouettes? Can a shadow have a reflection? Is the shadow of a spinning sphere stationary, or does it also spin? Can we see in complete darkness?
These are just a few of the intriguing and perplexing questions that Roy Sorensen attempts to answer in this remarkable book, which will be of interest not just to philosophers and psychologists but to anyone who has retained some sense of the magic that shadows have for children.
Sorensen begins by distinguishing shadows from silhouettes, focusing our thoughts by posing the riddle of the double eclipse. If Earth had two moons, a smaller but nearer one, Near, and a farther but bigger one, Far, then they could, by lining up between Earth and the sun, produce a double eclipse, with Far eclipsing the sun and Near eclipsing Far. In such circumstances, would we be seeing Near or Far when we viewed the double eclipse?
Most people are inclined to say Near, because it is in front of Far and they assume that one cannot see an opaque body when it lies behind another. That is true when we see an opaque body in virtue of its reflecting light into our eyes, but in an eclipse we see the eclipsing body in virtue of its blocking light that would otherwise have entered our eyes.
Sorensen argues that when we view the double eclipse we must in fact be seeing Far, and that we see it by seeing its silhouette. He contends that Far's silhouette is to be identified with its surface that is nearest to the sun - the surface that blocks the sun's light from us.
We see a silhouetted object by seeing its surface that is furthermost from us, because it is that surface that is causally responsible for blocking light from our eyes. This is exactly the reverse of the case with objects that are seen in virtue of their reflecting light into our eyes, which we see by seeing their surface nearest to us.
As for shadows, Sorensen points out that we should not confuse them with their two-dimensional cross-sections that are cast upon the surfaces of objects lying in their way, since shadows are really three-dimensional in shape. According to Sorensen, shadows are real, but are not "positive" beings, since they are absences of light - "holes in the light" - created by opaque material objects when they block light that is incident upon them.
Although Sorensen follows most philosophers in adhering to a causal theory of perception, he still maintains that we can literally see shadows, since he rejects the commonly accepted view that absences (or "privations") cannot be causes. However, whereas he allows that one may see an object by seeing its silhouette, he denies that one may see it by seeing its shadow, on the ground that the silhouette, but not the shadow, is a part of the object. The shadow is only an effect of the object and one cannot, he thinks, see an object by seeing its effects. He must therefore presumably deny, counterintuitive though it may seem, that one can really see an object by means of a television relay.
Sorensen denies that we can feel shadows - so that talk of "feeling a cool shade" cannot be construed literally - on the ground that temperature is possessed only by things containing molecules in motion. This, I think, is one of his rare slips of a scientific nature, since modern physics apparently allows that even the vacuum has a temperature.
He also denies that shadows can have any colour other than black, contending that we need to distinguish shadows themselves from the "light pollution" that may sometimes penetrate and fill them, rather as extraneous matter may sometimes penetrate and fill a hole in a material object. Sorensen coins a snappy new word, "filtow", for the kind of body of coloured light, produced by a filter, that may overlap or even coincide with a shadow.
Sorensen's book is endlessly fascinating and thought-provoking. No one who reads it will agree with everything that he says, but everyone will come away much the wiser for having read it.