  One of the fundamental problems with the interpretation of Quantum Mechanics,
according to Bohr, is the fact that "our usual description of physical
phenomena is based entirely on the idea that the phenomena concerned may be
observed without disturbing them appreciably". Furthermore in his articles
discussing the subject Bohr argues that the action of the probe will be
affected by the system and inasmuch the system will be affected by the probe.
Specifically in Gedanken experiments he tests the wave-particle duality of the
system and implicitly assumes that the probe is also a quantum system. A
universal character can only be attributed to Quantum Mechanics provided a
complementarity relation is also valid for the probe. As a consequence the
state system-probe becomes entangled. In the past fourty years complementarity
tests have been proposed. However, much less attention has been paid to the
study of an arbitrary probe system. In the present contribution, we fill in
this gap and show that the key ingredient for the quantum-classical transition
is not necessarily the information generated by the system-probe interaction
but rather by its accessibility. Our results have been successfully tested in
the interferometric experiment. Our results also allow for a simple physical
interpretation of the physics of Ramsey Zones where one photon (average)
interacts with a two level atom in a classical manner, i.e., no entanglement is
generated.
