Bhagavad Gita Verse 51, Chapter 18

buddhayaa vishuddhayaa yukto dhrityaatmaanam niyamya cha |
shabdaadeenvishayaamstyaktvaa raagadveshau vyudasya cha || 51 ||
 
Endowed with a pure intellect, and regulating oneself with fortitude, giving up sense objects beginning with sound, and eliminating like and dislike.
 
buddhayaa : with intellect
vishuddhayaa : pure
yuktaha : endowed
dhrityaa : with fortitude
aatmaanam : oneself
niyamya : regulating
cha : and
shabdaadeen : beginning with sound
vishayaan : sense objects
tyaktvaa : give up
raagadveshau : like and dislike
vyudasya : eliminating
cha : and
 
In this and the next two shlokas, Shri Krishna illustrates the lifestyle the monk who has entered the stage of sanyaasa or renunciation. The urge to perform a lot of physical actions is low for such a person, otherwise it is impossible for him to even enter a monastic life. Now, the focus shifts towards regulating the mind. Since the monk has already eliminated his strongest material desires, he has to focus on the ones that are weaker, with a view of completely eradicating them as well.
 
Such a sanyaasi has a buddhi or intellect which is vishuddha, extremely pure, free from any doubts and misconceptions about the nature of the material world. There is not even a trace of doubt about the existence of joy in inert objects. His dhriti or fortitude is saattvic, as we have seen earlier in the section on fortitude. He has complete mastery over his mind. He is able regulate his mind by his intellect, and prevent it from wandering into materialistic thoughts. Even if that does happen, his vision prevents those thoughts from causing desire. How is he able to do that?
 
This world is nothing but names and forms, and forms are comprised of five elements - space, air, fire, water and earth. Shabda adeen in this verse refers to these five elements. The sanyaasi is able to look at any object or person, see through the name and form, and understand that it is nothing but the five elements combined together. Realizing that any combination of these elements is nothing but Prakriti or matter, he is able to eliminate any sense of raaga or dvesha, like or dislike, towards that object or person.