This paper discusses what is fundamental in modern physics. We will start by arguing that completeness and universality are the two criteria or expectations that physicists have in mind for a theory of physics to be fundamental. To do this, we will make explicit what foundational assumptions are made in their conception of what is fundamental, one of which is that all other scientific theories can be reduced to the fundamental theory of physics, but not vice versa. The fact that this reduction only goes one way distinguishes the fundamental from the non-fundamental. Then we will focus on one objection on how duality poses a problem for identifying the fundamental theory in this way, and even questions if there is a fundamental level at all. The last section will discuss possible responses to this objection.
Joey L