determines the structure of ordinary matter. It is a unique glory of physics that we demand [*precise, quantitative*]{} agreement between our calculations and reality, whenever comparison is possible. Enough said.

### Exploring Canonical Structures

The fact that QCD is a tight, conceptually defined theory implies that the structures it produces are canonical.

Let me explain what I mean by this, by way of an analogy with black hole physics. A feature of black holes that makes them especially fascinating and attractive is that their theory is so clean. The key to this cleanliness is that, as Wheeler put it, “black holes have no hair”. Specifically, classical general relativity produces unique predictions for the properties of a black hole, given only its mass and angular momentum. This is quite different from stars or ordinary matter, whose structure depends on many parameters including elemental composition, temperature, and processing history. The canonical, nearly parameter-free structure of black holes means that it is worthwhile to study solutions of the equations that govern them minutely, because these solutions are sharply delimited.

In QCD the structure of all the hadrons is canonical, and the structure of protons and neutrons is especially so since they are stable 