  In this work we show that the null result of the Michelson-Morley experiment
in vacuum is deeply connected with the notion of time. It can be deduced
without any mathematics only from the assumption that all good clocks can be
used to measure time with the same results, independently of the machinery
involved in their manufacturing.
  A second important assumption, intrinsic to the very notion of time, is that
clocks measure time in the same way in different frames, i.e., the notion of
time is the same in all inertial frames. Under this assumption, we point out
that the "postulate" of constancy of the "two-way" speed of light in vacuum in
all frames independently of the state of motion of the emitting body is also
strongly related to the concept of time, together with the existence of a limit
speed in the "rest frame". This postulate simply results from the construction
of clocks where tic-tacs are made by objects traveling with the limit speed.
