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You cannot make a polygon with one or two sides: the first polygon is the triangle, which has three. One and two thus, in a sense, only exist in potential: they cannot take shape. But one plus two is three, so three gives one and two shape and makes the potential, actual. Indeed, the ancients didn't even consider one to be a number at all: Euclid, for example, says that number is the "multitude made of units," [Elements VII, def. 2] thus making one the measure of number and so beyond number. In that sense, one is doubly potential: it is number in potential and shape in potential. Two occupies a middle ground, being number in actuality but shape in potential. Three is finally what is both number in actuality and shape in actuality.

This is the kind of thing Porphyry talks about when he says that because "incorporeal forms and first principles could not be expressed in words, [the Pythagoreans] had recourse to demonstration by numbers." [Life of Pythagoras XLIX] So, metaphorically, one is heaven, spirit: that which is beyond and ever unreachable, even as it acts as a template. Two is earth, matter: both imminent in a sense and beyond in a sense, being infinitely divisible and never properly graspable. Three is the combination of the two, the things that exist from them, finally actual and sensible.

And the Neoplatonists loved to read these things into myth. Zeus is heaven, as the seed of all things. Maia is earth, that which receives and so gives form (but not form itself). Hermes is the result of their union, bringing the potential into actual, and so mediating between heaven and earth, and heralding the intelligible to the sensible.

Thus Zeus is spirit is one, Maia is matter is two, and Hermes is things is three. It is no mistake that Hermes is the patron of storytellers, for stories must have a beginning, middle, and end—three parts—in order to be complete; similarly, he is called thrice-greatest, because he brings perfection or completion or form to that which comes before.

Now, consider that you, yourself are a product of heaven and earth, possessing a spiritual soul and a material body. That means you are that which gives actuality to the potential. That means you are the mediator between potentials. That means you, yourself, are Hermes. When the Hermeticists say that Thrice-Greatest Hermes is their teacher, what they are really saying is that they are self-taught: truth does not come down from on high, it comes from within.