The mem, the letter of "water" (mayim) symbolizes the fountain of the Divine Wisdom of Torah. Just as the waters of a physical fountain (spring) ascend from their unknown subterranean source (the secret of the abyss in the account of Creation) to reveal themselves on earth, so does the fountain of wisdom express the power of flow from the superconscious source. In the terminology of Kabbalah, this flow is from keter ("crown") to chochmah ("wisdom"). The stream is symbolized in Proverbs as "the flowing stream, the source of wisdom."
In particular, we are taught that there are thirteen channels of flow from the superconscious source to the beginning of consciousness. These channels correspond to the Thirteen Attributes of Mercy revealed to Moses at Sinai, as well as to the thirteen principles of Torah exegesis, the (superrational) "logic" of Torah.
The mem is the thirteenth letter of the alef-beit. In Kabbalah we are taught that "thirteen mems," as it were, appear in the "primordial air," the (outer) "space" into which the letter lamed soars. Each attribute of mercy is in fact a contraction of relatively Infinite wisdom, at the level of the superconscious ("waters that have no end"), in order to channel and reveal a flash of wisdom on to the "screen" of consciousness. Conscious wisdom draws its points of insight primarily from that attribute of mercy referred to in Torah as "He retains kindness for thousands of generations," whose initial letters spell the Hebrew word for "stream, "the first word in the previously quoted phrase, "the flowing stream, the source of wisdom."
In at'bash, mem transforms to yud, the point of wisdom or revealed insight, the drop of water emerging from the fountain of the mem.
The words for "one" (echad) and "love" (ahavah) both equal thirteen, the secret of the letter mem. The closed, final mem, the source of the fountain of wisdom connected and included within its subterranean, superconscious source, corresponds to the secret of echad, "one." The open mem, from which emerges the point (yud) of conscious insight, is the first manifestation of love (i.e., will to cling to another) in the soul. The connection between the two fountains of the mem, the "closed" fountain and the "open" fountain, is by the power of the Thirteen Divine Attributes of Mercy. This is the secret of G-d’s Essential Name Havayah - the "Name of Mercy." The Name Havayah equals 26 = 2 times 13, the union of "one" and "love," the power to draw into consciousness the wisdom of Torah.
The open mem - a square with a small opening at its lower left corner.
The final mem - complete square.