In quantum mechanics non classical trajectories are allowed with a probability: $$ P\{ x(t)\} = {e^{ - {{S\{ x(t)\} } \over h}}} $$ (1) where the time is euclidean (t ---> it) and $$ P\{ x(t)\} = \int\limits_{ - \infty }^\infty {L[x(t),\dot x(t)]} $$ (1) When h goes to zero, the probability peaks at the minimum of the action , the fluctuations are damped and one recovers classical mechanics. Quantum field theories deal with the fluctuations of fields around the classical configuration, governed by a probability distribution: $$ P\{ \phi \} = {e^{ - {\rm{ }}{{S\{ \phi \} } \over h}}}, $$ (2) where $$ S\{ \phi \} = \int\limits_{ - \infty }^\infty {L\{ \phi (x} ),{\partial _\mu }\phi (x)\} . $$ (2)