  The concept of a Shear Transformation Zone (STZ) refers to a region in an
amorphous solid that undergoes a plastic event when the material is put under
an external mechanical load. An important question that had accompanied the
development of the theory of plasticity in amorphous solids for many years now
is whether an STZ is a {\em region} existing in the material (which can be
predicted by analyzing the unloaded material), or is it an {\em event} that
depends on the loading protocol (i.e., the event cannot be predicted without
following the protocol itself). In this Letter we present strong evidence that
the latter is the case. Infinitesimal changes of protocol result in
macroscopically big jumps in the positions of plastic events, meaning that
these can never be predicted from considering the unloaded material.
