We show how to generalize the classical duals found by Gabadadze {\it et al}
to a very large class of self-interacting theories. This enables one to adopt a
perturbative description beyond the scale at which classical perturbation
theory breaks down in the original theory. This is particularly relevant if we
want to test modified gravity scenarios that exhibit Vainshtein screening on
solar system scales. We recognise the duals as being related to the Legendre
transform of the original Lagrangian, and present a practical method for
finding the dual in general; our methods can also be applied to
self-interacting theories with a hierarchy of strong coupling scales, and with
multiple fields. We find the classical dual of the full quintic galileon theory
as an example.Comment: 16 page
