The ability to make signs is not unique to humans, but humans are sign-making virtuosos. Our outsized ability to make signs is one of the things that separates us from all other animals. Sign-making is the basis of language and is integrally related to what we call “intelligence” and “culture.”

A sign is nothing more than something that “stands for” something else. The letters on this page, for example, “stand for” sounds. It has long been recognized that there are several kinds of signs. The philosopher Charles Pierce, for example, conceived of a very useful typology of signs:

You can categorize almost any sign using Pierce’s handy typology. But only “almost any.” There are some interesting exceptions, and the most interesting among them are photographs.

We don’t usually consider photographs signs of anything. They don’t “stand for” something. Rather, they “depict” something. A photograph of you on your wedding day is not a “sign” of you; it’s you, or at most a “picture” of you.

But you have to admit that some photographs do “stand for” something. Take the photograph below.

Black Power Salute

It “depicts” three men, two of whom are holding their fists in the air. But because it is well known, it has come to “stand for” a wide variety of things. From most specific to least, it "stands for": a) the Mexico Olympics of 1968; b) the struggle of African Americans for civil rights in America; and c) defiance. As you look at this photograph, all of these things—referents—reflexively come into your “mind's eye.” The photograph, then, is definitely a sign. But is it an index, an icon, or a symbol?

You might be surprised to learn that photographs are, in fact, indexes. We tend to think of photographs as produced by photographers the way that paintings are produced by painters. But that’s not really correct. Once the photographer has lined up the shot and released the shutter, the image is mechanically produced by the interaction of light, the photographed object and the photographic plate. The first bounces off the second and imprints itself (so to say) on the third. Where there’s smoke, there’s fire; where there is light, an object, and a photographic plate, there is a photograph.

But photographs are also icons. After all, they plainly resemble the things they “stand for.” The men “look like” men; the fists “look like” fists; the gloves “look like” gloves. But even here there is some slippage. One might sensibly say that the photograph “looks like” the fight for civil rights, but it would be hard to say that it really “looks like” the concept of defiance. Defiance is so abstract that it doesn’t “look like” anything. Nonetheless, the sign is present in our minds, resting somewhere between icon and symbol.

Though photographs have a symbolic aspect, they don't rest easily in the category of “symbols.” This is primarily because they are not really arbitrary. We couldn’t substitute, say, the letter “b” for any element of the photograph above and expect for it to “mean” the same thing. If we replaced each figure with “b” the photograph wouldn’t mean anything at all, other than “I’m abstract art; try to understand me.”

So that's why the famous photographs explored in this project are called “mechanical icons.” They are “mechanical” because they are produced through the workings of a law-like physical process; they are “icons” because they resemble to one degree or an other the things they “stand for.”