  A seminar given about 30 years ago by Ruben Aldrovandi motivates this text
where some reflexions about constructing theories that modify General
Relativity are made. Two particular cases, the Brans-Dicke and Unimodular
Gravity ones, are discussed, in a quite qualitative way, showing on how they
can address some of the most outstanding problems of General Relativity,
specially the transplanckian physics and the cosmological constant problem.
