The main answers here have been "photography is a physics phenomenon" of light and some receptor. Which is like saying the essence of a tree is wood. If people had simply noticed that light can leave a latent image on light sensitive material and remarked, "there's an interesting physical process", we wouldn't have photographic images today. But they didn't do that. They immediately transcended the obvious physical process into something useful driven by human intentions. And they applied those intentions to such ideals as changing the world. So far then, we have physical process + human intention = idealized outcomes. All three of those terms comprise "photography" up through the 20th century. Since each term has many possible values, each practitioner finds its essence in alignment with their ultimate understanding. The greats of history found profound essence, others find essentially nothing but the physical properties of light on a sensor. If this were not so, each of the greats when asked about photography would have merely said, "it's light hitting a sensitive medium." Or words to that effect just describing the physical, unanimated process.
If you are totally happy with photography as a mere physics process, that's your belief and it's perfectly valid. But just know, that such wasn't the feeling of the acknowledged masters in the field. I'm simply collecting these beliefs to see how they compare to the past.