  Starting with a down to earth interpretation of quantum mechanics for a free
particle, the disappearance and reappearance of interference in the 2 slit
problem with a detector behind one are treated in detail. A partial
interpretation of quantum theory is employed which is simple, emphasizing
description, yet adequate for addressing the present problem.
  Given that the eigenvalue equation is essential to predict a free particle's
probability of collision, it is argued that there is equal need for a realistic
theory to describe its possible motion. Feynman's point-to-point space-time
wave packet is put forth and used as the appropriate description of the
field-free motion between collisions.
  For a particle in a conventional 2-slit experiment with attempted detection
behind one, the disappearance of interference is explained - both when the
detection succeeds and when it doesn't. Also a definite prediction is made,
when the inter-slit distance is reduced, of where the first signs of
interference should appear on the detection screen.
