As we explore the instrumentality that leads to the creation of the worlds of division and fragmentation, mind, life and matter, we see that while the Supermind holds the complete and integral Truth of Oneness while also recognizing the individual constituent parts, it needs some way to effectuate a break into the component pieces to allow the interplay to take place. This conduit is the overmental consciousness.
The Overmind, while aware of the Unity, has a more or less screening or filtering action, taking the full glare of the sunlight of the supramental level and diffusing the light down to something that the lower levels can actually bear. And in doing this, we see the first signs of the division or bifurcation that we are calling the Ignorance, where mind, life and matter have now lost the inherent awareness of the Oneness of which they are a part, and now act as if they are independent separate actors having to succeed on their own against the rest of creation.
In the Supramental level, the poles or opposites are nevertheless still two sides of the same reality, inextricably linked together and One in essence and reality. At the overmental level we see the separation of these powers and they begin to carry out somewhat different agendas.
Thus we see the big philosophical divergences, Purusha, the witness consciousness and Prakriti, the active nature, One and Many, Personality and Impersonality all arise as this transition takes place. These separations and distinctions are actually valuable as they allow new and untested expressions to occur and bring about variety and change to the universal action.
Depending on the starting point these distinctions lead eventually to different ways and means of evolution, development and spiritual focus; but in the end, since they are all dual aspects of one Truth, they must all re-unite and rediscover their Unity as we evolve toward the supramental status of consciousness.
reference: Sri Aurobindo, The Life Divine, Chapter 28, Supermind, Mind and the Overmind Maya