What is Devi Tattva

There are 36 elements present in us. First is the Earth element, second Water, Fire, Air, Ether and you go on like that and the last one, the most refined is Shiva Tattva. One is finer than the other. Earth is the most gross, Shiva is the finest, the most subtle, the subtlest of all. So in these 36 elements which are present in us, Devi is also a Tattva. 
The last and finest is Shiva. And how is Devi present in us? It is present in many forms, in many emotions. When you see some brightness in someone, that is Devi manifested. When you see total peace, essence of deep peace, peace in the heart, not just the temporary peace, total fulfillment, satisfaction, that is Devi. And when you feel restlessness, that is also Devi. So if someone is agitated, you stand back and you say ‘Oh, Devi has manifested as agitation in this person.’ The Devi is manifesting in everyone as hunger. Someone is hungry, that hunger is Devi manifested. Someone is confused, that confusion is Devi manifested.  
When we chant the Durga Saptashati (referring to the 700 verses of the Devi Mahatmayam from the Markandeya Purana, dedicated to praising the Mother Divine), there is a verse which says, ‘Ya Devi sarva-bhuteshu bhranti rupena samstitha; Namastasye namastasye namastasye namo namaha’ (meaning: O Divine Mother! I bow down to You who are manifest as Bhranti – the element of delusion – in all living beings).
It means that it is the Mother Divine who is present as delusion within you.
Navratri is the occasion where the bridge between the subtle and the gross world is more obvious than other times. Navratri is when you recognise that the universe is governed by the subtle force. The mind is governed by the subtle force, honouring and adoring that subtle force is Navratri. The mind is duality. Mind means faith & doubt, happiness & sadness, connectivity & disconnectedness. That is the mind. The big mind winning over the small mind, that is Devi Shakti.