Chapter 6, Verse 34
Part of 6: Dhyān Yog — ध्यानयोगचञ्चलं हि मनः कृष्ण प्रमाथि बलवद्दृढम्। तस्याहं निग्रहं मन्ये वायोरिव सुदुष्करम्॥
chañchalaṁ hi manaḥ kṛiṣhṇa pramāthi balavad dṛiḍham tasyāhaṁ nigrahaṁ manye vāyor iva su-duṣhkaram
The mind is indeed restless, turbulent, strong, and unyielding, O Krishna; I consider it as difficult to control as controlling the wind.
The mind constantly changes its objects and so it is ever restless. Krishna is derived from Krish which means to scrape. He scrapes all the sins, evils, and the causes of evil from the hearts of His devotees. Therefore He is called Krishna. The mind is not only restless but also turbulent or impetuous, strong and obstinate. It produces violent agitation in the body and the senses. The mind is drawn by the objects in all directions. It works always in conjunction with the five senses. It is drawn by them to the five kinds of objects. Therefore it is ever restless. It enjoys the five kinds of sensobjects with the help of these senses and the body. Therefore it makes them subject to external influences. It is even more difficult to control it than to control the wind. The mind is born of Vayutanmatra (wind rootelement). That is the reason why it is as restless as the wind.