1 Gita Sloka Every Day - Chapter 18 - Moksha Sanyasa Yoga - Sloka 33

The entire content of this mail is from Shri V N Gopala Desikan's  Srimad Bhagavad Gita, published by Vishishtadvaita Research Centre, Chennai and The Bhagavad Gita by Swami Chidbhavananda published by Ramakrishna Mission

The grades in the firmness of mind are being described now:

The Three Gunas Give Impetus to Karma - 18-40

1 Gita Sloka  Every Day - Chapter 18 - Moksha Sanyasa Yoga - Sloka 33

Dhrutya yayaa dhaarayate manah pranendriya kriyah I

Yogena avi abhicharanya dhrutih sa partha sattviki II sloka 33

धृत्य यया धारयते मनः प्रनेन्द्रिय क्रियः I 
योगेन अवि अभिचरण्य धृतिः स पार्थ सात्त्विकी II श्लोक 33

The unswerving firmness by which, through yoga, the functions of mind, the prana and the senses are regulated, that firmness, O Partha, is sattvika.

Dhriti means firmness. IT also means fortitude or resolve. This virtue is indispensable to him who has chosen to live a spiritual life. Again as one advances in the ethical and spiritual life dhriti in that one becomes pronounced in details. The diverging sun beams may be made to converge again. That gathered up brilliance, directed at anything, is capable of providing greater revelation about that thing. Dhriti is akin to this. Knowledge and action converged into one and increased in capacity is termed as firmness. Yoga fosters dhriti. As one advances in yoga, fortitude develops in that one as a matter of course. The mind, the prana, and the senses of a yogi become increasingly acute and efficacious. The benign part of this great potentiality is that these sharpened faculties are all utilised in teh service of the paramatma. That is, all feelings and cogitations of the mind are devoted to adoration of the Almighty. Prana is the life energy. The aspirant has no other aspiration in life than to direct it to the glory of the Lord. The functions of the senses are all dedicated to his worship in as many ways as possible. 

A thread is made of innumerable fibers. Even so dhriti is the outcome of the combined sublimated activities of the mind, prana and the senses. The fibers at the end of a thread ought to be twisted to converge in order for it to pass through the eye of a needle. But if that eye is blocked with dirt, then the pointed thread would bend instead of passing through it. And this is due to lack of stiffness in the thread. But the dhriti of a yogi is as pointed as the end of the thread and as stiff and penetrating as a sharp needle. Such a carefully cultured shriti is dedicated by the yogi to the benign invocation of the Lord. It is in no way prostituted for any other purposes. As the needle of a compass always points north, the resolve of the yogi is ever directed to the Iswara. It is therefore unswerving. This dhriti is sattvka in its make and leads the sadhaka to the supreme.