Tasya hetuh avidya. (II 24)
Tasya = its (of the union); hetuh = cause; avidya = ignorance.
Its cause is the lack of awareness of real nature.
We saw, in previous Sutra, the purpose of coming together of purusa and prakrti is realization by purusa of its true nature. And we illustrated it through the example of the bright sun covered by clouds. We can't feel the brightness of the sun when it is covered. As soon as the clouds disperse, we get the pure sunlight.
Maya, the great illusion is like clouds that cover our consciousness. Due to this power of Maya we become ignorant. This ignorance (avidya) causes involvement of Purusa with Prakrti. Otherwise how such a Purusa, with pure consciousness will come to the stage of bondage? The question is often asked – no doubt Prakrti is the school through which Purusa evolves but how, pure, reaches that state from where it has to evolve?
But that is the power of Avidya (ignorance). This Avidya is the root of all the misery and coming out of this avidya leads to freedom, the moksha.
Our classical literature is full of episodes depicting how even the wise persons got trapped by the power of Maya and lost control on themselves. Sadhana is progressively gaining that WISDOM (VIDYA) which will free us from the bondage of Maya, the illusion.