  A simple unified closed form derivation of the non-linearities of the
Einstein, Yang-Mills and spinless (e.g., chiral) meson systems is given. For
the first two, the non-linearities are required by locality and consistency; in
all cases, they are determined by the conserved currents associated with the
initial (linear) gauge invariance of the first kind. Use of first-order
formalism leads uniformly to a simple cubic self-interaction.
