Several passages in the Torah deal with the laws of ritual contamination; in contrast, we find only one which relates to purity. It tells us that even though, under most circumstances, water can become contaminated, a spring or a cistern cannot become contaminated.
The verse clearly informs us that: “Only a spring or a cistern, a gathering of water, shall remain pure...” (Leviticus 11:36)
We conclude from this verse that the water in the cistern or spring is impervious to the effects of ritual contamination. It is essential to note the distinction. Not all water has this special property. The Torah clearly stipulates that only water which flows directly from nature, so that it remains a part of the earth is pure to the extent that it cannot be affected by ritual impurity. As a practical consequence of this distinction, let us say that a contaminated object fell into a container of food or beverage. All of the food or drink, together with the vessel itself, becomes ritually impure. In contrast, should exactly the same impure object fall into a natural spring or into a cistern, neither the spring nor the water in it becomes contaminated. This “earth-bound” water, still a part of the earth, is impervious to contamination.
To sum up, we find that the general rule is that any water which has not been cut off from its source in nature cannot become impure, just as the earth itself cannot become contaminated. What is more, this naturally flowing water has the additional property of being able to absolve the impurity of a person or an object which is totally immersed in it.
If we reflect on the matter, we perceive that man instinctively associates a state of cleanliness with water and purity. Consequently, when a person immerses himself in a mikveh, it is not difficult to call up a positive association with purity and cleanliness as he comes into contact with the water. He will easily interpret this sensation as a spiritual cleansing from the impurity which clung to him.
In traditional Jewish sources we find that the statement that “...in the act of immersing in a mikveh there is an allusion that it should cleanse his soul of all sin, just as it is the nature of water to cleanse anything that is laundered in it.” (Sefer Hachinuch, Commandment 175). In this moment of immersion, Man brings to a climax a process of personal change. By his inner nature, a person aspires to purity and refinement. The process of achieving this goal utilizes the natural perceptions in order to awaken a sensation of spiritual cleansing.
If we analyze the Torah texts which deal with this topic, we will come to a more comprehensive understanding of this precept.
We find another instance of ritual immersion in the Book of Exodus, 29:4.G-d commands Moses: “Aaron and his sons you shall bring near the entrance of the Tent of Meeting, and you shall immerse them in water.”
Rabbi Samson Rafael Hirsch comments on these words:
Through this immersion, Moses, as the head of the nation, elevates Aaron and his sons, who are consecrated to become priests, and dissolves their ties with their past. In this case, Aaron and his sons were not cleansing themselves from ritual contamination. Even before the ceremony began, Aaron was required to be in a state of ritual purity. Nonetheless, G-d commanded that he and his sons be immersed in the mikveh. Through this act, Aaron's personal status was transformed for all time. He became a priest, entitled and privileged to enter the Sanctuary and to serve G-d there. What was more, his descendants would inherit this higher status of priesthood for all generations to come.
A striking expression of Aaron's privileged status is found on the Day of Atonement, Yom Kippur. On this holy day, the High Priest was allowed – nay, commanded – to enter the Holy of Holies, where the Ark of the Covenant was stored. It was the most sacred moment of the service performed on the holiest of days. What happens during this service, as prescribed by the Torah?
Before entering the Holy of Holies, the High Priest immersed himself in a mikveh and changed his garments. This act of immersion was not required in order to remove a state of impurity. Obviously, The High Priest was already tahor – ritually pure. Were it not so, he would not have been allowed to perform the ritual service of the Day of Atonement up to this point. Rather, this specific act of immersion served as a preparation for the High Priest's attainment of yet a higher status, that required of one who is about to enter the Holy of Holies.
Another instance of a change of status through immersion in a mikveh is the case of a gentile who converts to Judaism. Again, in this instance, we note that there is no question of removing ritual impurity, since a gentile has no status regarding in this matter. He is neither ritually pure nor impure; the laws of ritual contamination do not apply to him. His immersion in the mikveh in this case is the culmination of the process of his elevation from the status of gentile to that of a member of the Jewish People. Conversion is an extended process, with multiple requirements. It reaches its climax at the moment of immersion in the mikveh. Henceforth, Jewish Law regards this individual as a full-fledged Jew, with all the obligations thereof.
How is it that immersion in the waters of the mikveh can alter a person's status? Is there some chemical process involved which effects the transformation?