To order the levels of description in physics, and find out what makes one layer more fundamental than another, the concept of emergence helps. If one phenomenon is the result of another, and clearly emerges from it, that places them in order, and shows which runs deeper. But here it is suggested that alongside emergence, we need to think about each layer explaining the one above it. Explanation is a concept that comes very much from human thinking, but then so does some of the surrounding landscape. That's because the levels we identify in physics are in the first place, to some extent, levels of conceptual understanding. The view here is that ‘fundamental’ has an objective element involving emergence, and a parallel subjective one, involving explanation. The two give the same directional ordering of the layers.
Jonathan Kerr