In general the first principle is a basic assumption that cannot be deduced any further. Related to different fields of human activity there are different definitions of first principles, for example for engineering those are the laws of physics. Often great innovation in science/engineering happens when the new idea is not build on top of the current state of the art or commonly accepted technology. Instead the problem is initiated from those first principles or in other words "what we know for sure" and re-build from there.
So, what are the first principles known so far in computer vision, particularly in object detection. Are there fundamental "can do" and "cant do" that take its roots and proofs in computer science, physics, mathematics?