  The following two papers form a natural development of a previous series of
three articles on the foundations of quantum mechanics; they are intended to
take the theory there developed to its utmost logical and epistemological
consequences. We show in the first paper that relativistic quantum mechanics
might accommodate without ambiguities the notion of negative masses. To achieve
this, we rewrite all of its formalism for integer and half integer spin
particles and present the world revealed by this conjecture. We also base the
theory on the second order Klein-Gordon's and Dirac's equations and show that
they can be stated with only positive definite energies. In the second paper we
show that the general relativistic quantum mechanics derived in paper II of
this series supports this conjecture.
