  Hawking and the author have proposed a class of singular, finite action
instantons for defining the initial conditions for inflation. Vilenkin has
argued they are unacceptable. He exhibited an analogous class of asymptotically
flat instantons which on the face of it lead to an instability of Minkowski
space. However, all these instantons must be defined by introducing a
constraint into the path integral, which is then integrated over. I show that
with a careful definition these instantons do not possess a negative mode.
Infinite flat space is therefore stable against decay via singular instantons.
