In the Tantric cosmology, the whole universe is perceived as being created, penetrated and sustained by two fundamental forces, which are permanently in a perfect, indestructible union. These forces or universal aspects are called Shiva and Shakti.
Tradition has associated a form to these principles, respectively that of a masculine deity and that of a feminine one. Accordingly, Shiva represents the constitutive elements of the universe, while Shakti is seen as the dynamic potential, which makes these elements come to life and act.
From a metaphysical point of view, the divine couple Shiva-Shakti corresponds to two essential aspects of the One: the masculine principle, which represents the abiding aspect of God, and the feminine principle, which represents Its Energy, the Force which acts in the manifested world, life itself considered at a cosmic level.
From this point of view, Shakti represents the immanent aspect of the Divine, that is the act of active participation in the act of creation. Maybe this Tantric view of the Feminine in creation contributed to the orientation of the human being towards the active principles of the universe, rather than towards those of pure transcendence.
Therefore, Shiva defines the traits specific to pure transcendence and is normally associated, from this point of view, to a manifestation of Shakti who is a somewhat terrible, personification of Her own untamed and limitless manifestation (such as Kali and Durga).
Owing to the fact that in a way, Shakti is more accessible to human understanding (because this regards aspects of life that are closely related to the human condition inside creation), the cult of the Goddess ( DEVI ) has spread faster and with more force.
This cult was combined with notions of the Shamkhya philosophy and has offered the premises necessary to the Maya doctrine that came later, as formulated by the sage Shankaracharya.
This possibility of combining, of interpenetration of two or more notions belonging to two or more spiritual systems is due to the fact that the Hindu philosophical systems are not isolated, closed systems, but complex and manageable doctrines, which may adjust and may be understood from different perspectives.
The Shamkhya philosophy promulgates a primordial cosmic duality. From the Tantric perspective, this "scheme" of creation is transformed, in the sense that the two cosmic principles are considered united, not separated. This is the fundamental difference introduced by Tantra, based on the conception that favors the unity between the two principles, opposed in appearance, but united in each act of creation.
Nevertheless, Tantrism confers to Shiva and Shakti the qualities of the masculine and feminine principles from the Shamkhya philosophy.
The idea of an act of fertilization at the divine, purely spiritual level comes to life and is admitted in the tantric conception, unlike the Shamkhya philosophy. The eternal and indestructible union between Shiva and Shakti gives birth to the whole Macro cosmos, in its stable, static aspect as well as in its dynamic one.
Thus, on one hand is presented the cosmic dance of Shakti on the lying body of Shiva. On the other hand, the two deities are pictured in what is called VIPARITA - MAITHUNA, meaning tantric sexual union. This sexual union is different from what is usually understood by the western traditions, in the sense that the man is immobile, while the woman, embracing him, assumes an active role during the sexual act.
In conclusion, Tantrism sees cosmic evolution as polarization within the Supreme Being, which is God, the Unmanifested Absolute, in its two fundamental aspects: static and kinetic.
Tantra symbolizes this aspect by comparing the couple Shiva and Shakti with a seed of wheat (CHANAKA). Such a seed is made up of two halves so closely linked that they seem one, and one single cover covers them. Symbolically, the two halves represent Shiva and Shakti, the cover represents Maya (the cosmic illusion). When the cover is on, the two halves are separated, but the cover goes off when the seed is about to germinate, thus unifying the two halves in one.