The end of Mes. Makkos discusses the halachos of giving malkos. The Ramban in last weeks parsha explains that "arba'im yakenu", the 40 lashes given, correspond to the 40 days it takes for an embryo to be considered fully formed and the 40 days of mattan Torah. The obvious question: Chazal understood the pasuk to mean only 39 lashes are given, not 40?
Maharal has a beautiful explanation. In each of the first 40 days starting from conception there is some aspect of a person which comes into this world, until at day 40 a fully formed embryo exists. On the first day it is the shoresh haneshama of the person, the core essence of a person's being, which is brought into the world. On all subsequent days various aspects of the body which clothe that core essence are added.
Each lash of malkos heals an aspect of the person which sin corrupted. There is, however, a limit to the damage sin can do. Every aspect of a person that comes into the world can become corrupted, but the shoresh haneshoma always remains pristine and pure. Therefore, only 39 lashes, never 40, are all that is needed to correct the flaws of cheit.