Proper Exposure is very debatable with a simple thought exercise. Imagine a green grass lawn, this would be your mid-tone. If there is a shadow over the lawn but the exposure is locked on the sunlit part, then you are going to get a much darker green in the image but is that exposure correct?
It would depend, if you had a person standing in the shade then you are probably going to want to open up and not expose to the right because the quality of light in the shadow is going to be poor, bluish (as the lawn in shadow is lit from the blue of the sky and not the sun), so the proper exposure is going to put the mid-tone of the sunlit grass higher on a histogram and risk blowing it out because otherwise you are going to be raising the shadows where you would have a lot of noise.
It gets funky, when you have two people on the lawn, one in the shade one in the light, then the correct exposure is probably going to be the original (a reflector/ fill-in flash would then be useful).
Take a black cat on a pile of coal, the final image is going to have a histogram where almost every tone is going to be humped to the left, but relying on a camera reading without exposure compensation is going to give a RAW where the histogram is humped in the middle.
The fundamental thing is that the photographer takes control, he/ she makes an active decision what they want in the final image and then exposes for that effect.