Honesty. Photography's core is honesty.
That can be broken, of course, but honesty is a big part of photography.
Let me explain with a couple examples:
1. Almost anyone who speaks any European language knows the name Jesus Christ. Also, almost anyone who speaks a European knows the name Adolf Hitler. But which of these two historical figures can you think of and say, "I know what that dude looked like."?
The same goes for Barack Obama and George Washington.
You may think you know what George Washington looked like, but you will never, ever really know, because he was painted. The person doing the painting could have been the most skillful artist in the world, but he might also want to get hired again. Therefore, that mole with the hair growing out of it? Nah, we'll ignore that. He isn't very fit? Well, we can adjust that a bit. And perhaps we will choose which wrinkles make him distinguished and which do not.
Yes, the same argument can be made for photography (we can photoshop wrinkles and use the healing brush to remove a pimple), but the difference is that we know we CAN get a 100% honest photograph.
So we will never know what Julius Caesar looked like, or King Henry the 8th, or Moses.
But we will always know what Margaret Thatcher, Adolf Hitler, and David Lee Roth looked like.
That is photography's core.