We have seen that mikvah entails two basic concepts, namely, water and the number forty. Both of these concepts are contained in a single letter, namely, the Hebrew letter mem.
The letter mem derives its name from mayim, the Hebrew word for water. Furthermore, the numerical value of the letter mem is forty. Therefore, it is not very surprising to learn that the letter mem is also said to represent the mikvah.
Another concept that we find associated with the letter mem is that of the womb. The closed (final) mem is the womb closed during pregnancy, while the open mem is the open womb giving birth. The numerical value forty, associated with mem, then also represents the forty days during which the embryo is formed.
Mem is the letter that represents transition.
In order to understand the meaning of this letter on a deeper level and see how it relates to mikvah, we must delve into a most interesting Midrash. The Prophet says (Jeremiah 10:10) "The Lord, G‑d, is Truth (emes)." The Midrash then gives the following explanation:
What is G‑d's seal? Our Rabbi said in the name of Rabbi Reuven, "G‑d's seal is Truth."
From this we see that the letter mem has a most interesting property. Alef, the first letter of the alphabet, represents the beginning. Tav, the last letter, represents the end. mem is the letter that represents transition.
We see this most clearly in the word emes itself. The first two letters, alef mem spell out em, the Hebrew word for mother. This is the beginning of man. The last two letters, mem tav, spell out met - the Hebrew word for death - the end of man.
Most important here, mem represents the concept of transition and change. alef is the past, and tav the future, so mem represents the transition from past to future. As such, it is the instant that we call the present.
Thus, when a person enters the mikvah, past and future cease to exist for him
The past is history and cannot be changed. We have no way of even touching the future. Therefore, the arena of action, where all change takes place, is the present. Symbolic of water, the essence of change, as well as the number forty, the essence of birth, the letter mem also represents the present, the transition between past and future - which is the arena of all change. On a deeper level, the transition from past to future also represents an aspect of birth. Indeed, one word for "future" in Hebrew is HaNolad, which literally means, "that which is being born." The womb in which the future is born is the present. This is the letter mem.
Thus, when a person enters the mikvah, he is actually entering the concept of the ultimate present. Past and future cease to exist for him. What he was in the past no longer counts. Even the forty days of formation are no longer an expanse of time, but a volume of water - forty sa'ah (totaling about 120 gallons). Then, when he emerges from the mikvah, he reenters the stream of time as if he were a new being.
[From Waters of Eden]