I think you&#x27;re both right. Carroll is claiming that in physics, there is one essentially <i>complete</i> layer of description that we fully understand: the layer consisting of the standard model (based on quantum field theory) plus general relativity. We don&#x27;t understand the smaller, more fundamental layers (string theory is one attempt at that, and there are others), and we don&#x27;t understand every last detail of how that layer gives rise to the larger, more familiar layers (we&#x27;ve only very recently even been able to deduce the mass difference between protons and neutrons based on standard model computational physics). But the point is that our understanding of this one particular layer is broad and solid enough to essentially guarantee that every other observable feature of real life can be traced back to a set of principles that we understand.