“The picture of life contrasted with the fact of life… All that is really peculiar to humanity … proceeds from this one faculty or power,” early photography advocate Frederick Douglass observed in contemplating the power of “aesthetic force.” But what is it that lends photography its singular power to capture and convey the facts of life? In On Photography (public library) — that same indispensable 1977 volume that presaged the dynamics of visual culture on the social web — Susan Sontag considers how photography differs from the other arts and what makes it a unique medium for human communication and consciousness. Her thoughts are doubly interesting to revisit decades later, when digital photography has lowered the barrier of entry so much that “everyone is a photographer,” as the aphorism goes, and as we find ourselves immersed in an ever-flowing stream of digital images flickering before our eyes faster than we’re able to contain them, let alone interpret them, in our minds. Today, as the photographic image becomes both more ephemeral (a string of information bits rendered on a screen, deletable and manipulable at the touch of a button) and more inescapably permanent (“The Internet is a copy machine,” Kevin Kelly tells us. “Once anything that can be copied is brought into contact with the Internet, it will be copied, and those copies never leave.”), Sontag’s meditation gets at the heart of what lent photography its increasingly affirmed status as the most powerful and far-reaching communication medium of our time.

Sontag begins by considering how photographs offer an assurance of permanence — of tangibility — far greater than the moving image, a sort of guarantee of experience that also explains why photos of vacations, progeny, and backstage passes populate Facebook feeds, as well as a sense of actuality greater than paintings and drawings:

To collect photographs is to collect the world. Movies and television programs light up walls, flicker, and go out; but with still photographs the image is also an object, light-weight, cheap to produce, easy to carry about, accumulate, store. . . .

Photographs are perhaps the most mysterious of all the objects that make up, and thicken, the environment we recognize as modern. Photographs really are experience captured, and the camera is the ideal arm of consciousness in its acquisitive mood.

To photograph is to appropriate the thing photographed. It means putting oneself into a certain relation to the world that feels like knowledge — and, therefore, like power. . . . Photographic images … now provide most of the knowledge people have about the look of the past and the reach of the present. What is written about a person or an event is frankly an interpretation, as are handmade visual statements, like paintings and drawings. Photographed images do not seem to be statements about the world so much as pieces of it, miniatures of reality that anyone can make or acquire.

But even if photographs imply an interpretation of the world, they invariably attest to the world — that particular world — existing in the first place:

The picture may distort; but there is always a presumption that something exists, or did exist, which is like what’s in the picture. Whatever the limitations (through amateurism) or pretensions (through artistry) of the individual photographer, a photograph — any photograph — seems to have a more innocent, and therefore more accurate, relation to visible reality than do other mimetic objects.

[…]

While a painting or a prose description can never be other than a narrowly selective interpretation, a photograph can be treated as a narrowly selective transparency. But despite the presumption of veracity that gives all photographs authority, interest, seductiveness, the work that photographers do is no generic exception to the usually shady commerce between art and truth. Even when photographers are most concerned with mirroring reality, they are still haunted by tacit imperatives of taste and conscience. . . .

In deciding how a picture should look, in preferring one exposure to another, photographers are always imposing standards on their subjects. Although there is a sense in which the camera does indeed capture reality, not just interpret it, photographs are as much an interpretation of the world as paintings and drawings are.

And yet despite their affirmation of reality — or, rather, a reality — photographs also prompt invisible narratives:

Photographs, which cannot themselves explain anything, are inexhaustible invitations to deduction, speculation, and fantasy.

Photography implies that we know about the world if we accept it as the camera records it. But this is the opposite of understanding, which starts from not accepting the world as it looks. All possibility of understanding is rooted in the ability to say no. Strictly speaking, one never understands anything from a photograph.

On Photography remains a must-read, increasingly so as the steady coevolution of technology and culture continues to complicate our relationship with the still image. Sample more of it here. Complement it with 100 ideas that changed photography and the history of photography animated, then revisit the best photography books of 2013.