It's not that arbitrary to me. The earlier work was sometimes physically-motivated, and sometimes even based on physics. For example using Lambertian reflection

It's not that arbitrary to me. The earlier work was sometimes physically-motivated, and sometimes even based on physics. For example using Lambertian reflection[1] for diffuse surfaces. But the key, in my view, to PBR is that the whole pipeline is based on physical principles and math, while the old renderers were not. So previous renderers would not bother properly normalizing the Lambertian coeffients for example, leading to violation of energy conservation. [1]: https://en.wikipedia.org/wiki/Lambertian_reflectance https://en.wikipedia.org/wiki/Lambertian_reflectance