  In this essay we propose that the theory of gravity's vacuum is described by
a de Sitter geometry. Under this assumption we consider an adjustment mechanism
able to screen any value of the vacuum energy of the matter fields. We discuss
the most general scalar-tensor cosmological models with second order equations
of motion that have a fixed de Sitter critical point for any kind of material
content. These models give rise to interesting cosmological evolutions that we
shall discuss.
