  In a purely relational theory there exists a tension between the relational
character of the theory and the existence of quantities like distance and
duration. We review this issue in the context of the Leibniz-Clarke
correspondence. We then address this conflict by showing that a purely
relational definition of length and time can be given, provided the dynamics of
the theory is known. We further show that in such a setting it is natural to
expect Lorentz transformations to describe the mapping between different
observers. We then comment on how these insights can be used to make progress
in the search for a theory of quantum gravity.
