In parshat Shemini we read about the death of Nadav and Avihu. In parshat Acharei Mot we read the follow-up to this story:
Hashem spoke to Moshe, after the death of the two sons of Aharon, when they approached Hashem, and died. Hashem said to Moshe: "Speak to Aharon your brother, that he not approach the holy place at all times..." (16:1-2)
Not only is Acharei Mot introduced by a reference to Shemini, but the Yom Kippur sacrifices specified in Acharei Mot are the same as those offered on the 8th day of the Mishkan's inauguration.
It would seem natural for Acharei Mot to immediately follow the story in Shemini. But there are five chapters of apparently unrelated material in between. What is this material doing there?
Looking at this material, we see it consists of four sections:
1. Kashrut of food
2. Impurity of a woman who has given birth
4. Bodily emissions
The common element between them is the centrality of purity and impurity. Sections 2 and 4 deal exclusively with such laws. And purity is the central theme in sections 1 and 3: non-kosher animal species are called "impure" species and it is emphasized that one who touches their carcass becomes impure; while to be diagnosed with leprosy is to be declared "impure".
It seems that the purpose of these laws is to establish ritual boundaries in different aspects of life. Nadav and Avihu's sin involved ignoring necessary behavioural boundaries in the Mishkan. The laws of purity and impurity, which establish such boundaries throughout life, are training to sensitize ourselves to such boundaries. Only after learning, practicing, and internalizing these laws can we be ready to return to the Mishkan. Acharei Mot, which teaches us how to properly enter the Mishkan as far as the Holy of Holies, must wait until we have prepared ourselves and there is no chance that Nadav and Avihu's mistake will recur.
Now, there are many laws of purity in the Torah and not all of them appear in our passage. Why where these particular laws chosen and others not? To answer this, let us look at a clear pattern in the themes of the laws that were chosen:
1. Substances entering the body
4. Substances leaving the body
*A leper is regarded as somewhat dead; see for example the description of Miriam's leprosy: "Let her not be like a dead person" (Bamidbar 12:12).
Sections 2 and 3 represent a person, from the beginning to the end of life. Sections 1 and 4 represent the limits between that person and the outside world. These four sections serve to define human existence in the dimensions of space and time. These are not the only laws of purity in existence. But they are sufficient to symbolize a system of purity and impurity which encompasses every aspect of our lives.