MSA
Having observed the discussion over on the "Unpublished let downs" thread, it is taken as read that to remain at or above MSA in IMC is "safe" yet to even consider a descent below it is "unsafe" unless on a published let down procedure (of whatever sort), precision or non-precision.
MSA is based on a vertical separation of 1,000 feet (apart from over mountainous terrain of course) and has been for very many years. The question is, why is it so? Why was that particular figure chosen?