Sattva-suddhi saumanasya aikagrya indriya-jaya atma-darsana yogyatvani ca. (II-41)
Sattva-suddhi = purity of sattva, purity of mind;
Saumanasya = cheerfulness; Aikagrya = concentration;
Indriya-jaya = control over senses; Atma-darshan
= self-realization; Yogyatvani = (and) fitness for; ca
From mental purity arises purity of Sattva, cheerfulness, concentration, control of the senses and fitness for Self-realization.
While physical purity reduces our attachment and fascination for the body, mental purity brings the qualities mentioned above.
What is mental purity? One can look at it from various angles. Getting rid of all such thoughts that obstruct the manifestation of divinity within is one aspect of mental purity. We have to consciously replace negative thoughts of lust, anger, jealously, greed and hatred etc with positive thoughts of brahmacharya, love and compassion. but ultimate mental purity is to get rid of all the vrittis, to go into deep silence transcending both good as well as bad thoughts. When we gain mastery over our thinking process then we can invite, at will, only desirable thoughts at desirable time. Hence concentration is the natural outcome of such type of mental purity. Then no unwanted thoughts can interfere in our process of thinking. We can keep depressing, negative thoughts at bay and be always cheerful. In place of senses governing us, we become the master and stop dancing to their tune. More than that we become pure enough, free from all the obstacles that inhibit the manifestation of divinity within. Like a clean mirror that reflects sun light in all its glory. To put it in Patanjali's words, "Tada Drastu Swarupe
Awasthanam" i.e. when free from vrittis, we get our true swarupa; we realize our true self.
Pranayama, Dhyana, japa etc are the methods to achieve this mental purity. Bhakti in its true sense is a sure way to mental purity. To begin with, to live a yoga way of life, we have to take proper, pure, sattvic food.