  Following P. A. M. Dirac's critique in ``Lectures on Quantum Field Theory''
of the usual formalism, I will discuss the role of the time parameter to solve
R. Haag's no-go theorem on the non-equivalence of the conventional
Schr\"odinger picture and the Heisenberg picture. This is possible by first
defining in a correct way the concept of the vacuum state at a given time in
relativity. I will also discuss some consequences such as the spectral
condition.
