[Note to readers: If you've heard any of this before, please let me know. I'm sure I'm not the first to come up with these ideas.]
The biblical purity system is one of the great enigmas of Israelite religion. It makes sense that the Israelites would want to keep that which is distasteful as far as possible away from the sanctum, and that an elaborate system might arise to ensure that this remained the case. But why should menstrual blood and semen be included among the sources of impurity, while urine, vomit, and mucus are not? Why skin diseases and not diseases of the eyes, for example, or the respiratory system? Why is semen impurity removed by bathing and waiting until evening, while birth impurity requires a waiting period of thirty-three to sixty-six days and a sacrifice, and death impurity requires sprinkling with the ashes of a red heifer?
Many attempts have been made to explain the psychology and/or theology behind the system. One popular interpretation, most thoroughly argued by Jacob Milgrom, is that impurity derives from an association with death, and the Israelite deity is a God of life. A human corpse is thus the source of the most severe type of impurity. Genital discharges represent the loss of potential life, because they were all believed to carry generative seed. Skin diseases, which are visibly degenerative, are closely associated with death, and thus generate impurity. Purification offerings (often translated "sin-offerings") involve the sprinkling of blood on various parts of the sanctuary, because blood is a symbol of life and therefore an appropriate "ritual detergent."
I think that this interpretation has merit, but it also has flaws. First, semen emitted in certain contexts may be best described as a "loss of potential life," but the substance itself is the source of impurity, and the substance itself is a source of life. The association of semen with death is most illogical when it is emitted during intercourse. Childbirth, similarly, causes impurity. Though this impurity seems to be associated with the bleeding that occurs during birth, the association with death still strikes me as a little bit strange. (I've heard it argued that bearing children involves a "partial death" for the mother because a human life is exiting her body, but I find this a bit of a stretch.)
Secondly, it is less than logical that blood issuing from human genitals -- a supposed source of generative seed -- symbolizes "death," while blood from a recently slaughtered animal symbolizes "life." This is the sort of reasoning that results from trying to fit the facts to an already established conclusion.
My own recent thought on the matter is that both the creation of life and its loss are potential sources of impurity as well as purity. These are most powerful forces, and they are rightly within the domain of the deity. Occurring in the natural course of human life, these forces generate impurity, but blood deliberately shed in a cultic context has the power to purify. There may be a connection between this idea and the law forbidding the consumption of blood: both an agent of life and a product of death, blood is not for human beings, but rather, for God.
A few additional thoughts: Most of the biblical purification rituals have a certain practical logic to them. Semen impurity cannot reasonably last more than a day or require a material sacrifice, since this would prevent couples from having sex and result in an undue monetary burden on men. Menstrual impurity, similarly, cannot reasonably require a sacrifice each time it occurs, but it must last longer than a day, since menstruation typically continues for about a week. Seven days is an appropriate duration for such impurity, being both the duration of a normal period and a biblical symbol of completion. Lengthy or irregular bleeding requires a seven-day wait after the bleeding has ceased in order to ensure that it does not begin again. Irregular discharges from both men and women require a sacrifice, which is reasonable, since by definition they occur less often than regular discharges; it is also understandable, given their mysterious and frightening nature and the fact that they may pose a health risk. Discharge of blood after childbirth may last for several days, resulting in a state similar to menstrual impurity. A lesser level of impurity follows, because of the lengthy period of infirmity and discharge that typically accompanies "natural" childbirth. Skin diseases require quarantine because they are contagious; this type of impurity cannot be removed before the patient has healed.
Other aspects of the system are less self-evident. One of these appears at the start of this week's Torah portion, where mothers are instructed to observe 7 + 33 days of impurity after giving birth to a boy, and 14 + 66 days for a girl. Those defending the system often point out that impurity may be rendered by the holiest of entities as well as the most profane, so this law is not necessarily misogynistic. I wonder whether females generate greater impurity because of their close association with the process of birth. This may not be either a "positive" or "negative" characteristic per se, but simply a certain type of power, which must be properly regulated.
Another perplexing law appeared in last week's additional reading, parshat parah. Death impurity can be removed only by waiting seven days and having a mixture of water and the ashes of a completely red (or brown) cow sprinkled on oneself with a willow branch. (This occurs both on the third day and the seventh.) Paradoxically, the priest who burns the cow, the man who does the sprinkling, and anyone who touches the ashes of the cow, become impure. It has been argued (reasonably) that the symbolic value of the completely colored cow is to increase the amount of "blood" in the mixture, this being a particularly severe form of impurity requiring a particularly concentrated form of the purifying substance. I would add that the very scarceness of such ashes increases the awe with which human death is regarded. But why should the ashes play a double role, rendering those with death impurity pure and those without it impure? I think that it is not the ashes themselves, but their association with death impurity that gives them the power to render the pure impure. Any association with the realm of human death, even with the substance that allows an individual to exit that realm, must have some consequence. Tellingly, a priest, who is forbidden contact with the dead, does not even sprinkle the purifying waters. For an individual whose purity is of such great consequence, even burning the cow that will later be used to treat death impurity requires a purification process.
The paradoxical red heifer ritual is the best illustration of the dual nature of blood, with its ability to impart or remove impurity, depending on context.