Parshas Ki Seitzei: Kli Yakar - Makkos, Two Behind and One in Front
ארבעים יכנו לא יוסיף - כי תצא כה:י
From this pasuk Chazal learn that for violating many aveiros in the Torah, thirty nine lashes are administered. The gemara (Makkos 23b) tells us that the Makkos are given in multiples of three, two thirds on the back and one third on the torso.
The Kli Yakar explains that the Makkos are given to help put us back on the right track. We sin because we forget the essential truths of life. The mishna in Pirkei Avos (3:1) says that by contemplating three basic things, a person will avoid being ensnared in sin. The first is that our humble beginnings are from a putrid drop. This, says the Kli Yakar, is something clearly before us and represents the one third of the Makkos that are given on our front side.
The other two things to consider is where we will end up, that being a place of dust, worms, and maggots, as well the fact that we be called to judgement before Hashem. These two facts are less apparent and are behind us, representing the two thirds of the Makkos given on the back.