  The logic of gauge theory is considered by tracing its development from
general relativity to Yang-Mills theory, through Weyl's two gauge theories. A
handful of elements---which for want of better terms can be called
\emph{geometrical justice}, \emph{matter wave}, \emph{second clock effect},
\emph{twice too many energy levels}---are enough to produce Weyl's second
theory; and from there, all that's needed to reach the Yang-Mills formalism is
a \emph{non-Abelian structure group} (say $\mathbb{SU}\textrm{(}N\textrm{)}$).
