  This short note is intended to review the foundations of mechanics, trying to
present them with the greatest mathematical and conceptual clarity. It was
attempted to remove most of inessential, even parasitic issues, which can hide
the true nature of basic principles. The pursuit of that goal results in an
improved understanding of some topics such as constrained systems, the nature
of time or the relativistic forces. The Sr\"odinger and Klein-Gordon equations
appear as conditions fulfilled by certain types of classical solutions of the
field equations although the meaning of quantum equations is not, even
remotely, exhausted by these cases. A part of this note comes from previous
works.
