  Einstein's reply to Weyl about the importance in General Relativity of the
identity of the sources of spectral lines is well know. We show that, already
in Special Relavitity, Einstein's definition of the unit of time from the
frequency of atomic emission is deeply incompatible with Poincare's classical
conception of time in his special relativity based on the duality "true
time-local time". By using Weisskopf's analysis of quantum nature of the
identity of atoms, we show that the concept of identity is implicit in
Einstein's principle of relativity. On the contrary, in Poincare's relativistic
logic, the use of the Lorentz transformations compensates the differences
(contraction of lengths and dilation of durations) that are real by principle.
The borderline "quantum-classical" passes therefore clearly between the two
conceptions of the clocks in special relativity.
