We show how a detailed analysis, mainly but not only based on the point of view of causality, of the usual definition of mechanical impulse of Classical Mechanics of free systems discloses theoretical unclear assumptions and practical applicability problems of very basic nature. We prove that Classical Impulsive Mechanics of constrained systems is not subject to these criticisms, being then a well founded theory. We propose then to step out of the concept of mechanical impulse acting on a free system and to consider impulsive aspects of Classical Mechanics as peculiar of constrained systems.