The third wonder of the Bodhisattva myth is that the first wonder (namely, the bisexual form) is symbolic of the second (the identity of eternity and time). For in the language of the divine pictures, the world of time is the great mother womb. The life therein, begotten by the father, is compounded of her darkness and his light. We are conceived in her and dwell removed from the father, but when we pass from the womb of time at death (which is our birth to eternity) we are given into his hands. The wise realize, even within this womb, that they have come from and are returning to the father; while the very wise know that she and he are in substance one.
This is the meaning of those Tibetan images of the union of the Buddhas and Bodhisattvas with their own feminine aspects that have seemed so indecent to many Christian critics. According to one of the traditional ways of looking at these supports of meditation, the female form (Tibetan: yum) is to be regarded as time and the male (yab) as eternity. The union of the two is productive of the world, in which all things are at once temporal and eternal, created in the image of this self-knowing male-female God. The initiate, through meditation, is led to the recollection of this Form of forms (yab-yum) within himself. Or on the other hand, the male figure may be regarded as symbolizing the initiating principle, the method; in which case the female denotes the goal to which initiation leads. But this goal is Nirvana (eternity). And so it is that both the male and the female are to be envisioned, alternately, as time and eternity. That is to say, the two are the same, each is both, and the dual form (yab-yum) is only an effect of illusion, which itself, however, is not different from enlightenment.
This is the supreme statement of the great paradox by which the wall of the pairs of opposites is shattered and the candidate admitted to the vision of the God, who when he created man in his own image created him male and female. In the male’s right hand is held a thunderbolt that is the counterpart of himself, while in his left he holds a bell, symbolizing the goddess. The thunderbolt is both the method and eternity, whereas the bell is “illumined mind”; its note is the beautiful sound of eternity that is heard by the pure mind throughout creation, and therefore within itself.
– Joseph Campbell. (1949. P 169-171). The Hero with a Thousand Faces. Fontana Press.