  The two-photon ghost interference experiment, generalized to the case of
massive particles, is theoretically analyzed. It is argued that the experiment
is intimately connected to a double-slit interference experiment where, the
which-path information exists. The reason for not observing first order
interference behind the double-slit, is clarified.It is shown that the
underlying mechanism for the appearance of ghost interference is, the more
familiar, quantum erasure.
