First among the qualifications required of the aspirant for Jnana, or wisdom, come Shama and Dama, which may be taken together. They mean the keeping of the organs in their own centers without allowing them to stray out.
I shall explain to you first what the ‘organ' means. Here are the eyes; the eyes are not the organs of vision but only the instruments. Unless the organs are present, I cannot see, even if I have eyes. But, given both the organs and the instruments, unless the mind attaches itself to these two, no vision takes place. So, in each act of perception, three things are necessary — first, the external instruments, then the internal organs, and lastly the mind. If any one of them be absent, then there will be no perception. Thus the mind acts through two agencies — one external and the other internal.
When I see things, my mind goes out, becomes externalized; but suppose I close my eyes and begin to think, the mind does not go out, it is internally active. But, in either case, there is activity of the organs. When I look at you and speak to you, both the organs and the instruments are active. When I close my eyes and begin to think, the organs are active, but not the instruments. Without the activity of these organs, there will be no thought. You will find that none of you can think without some symbol.
In the case of the blind man, he has also to think through some figure. The organs of sight and hearing are generally very active. You must bear in mind that by the ‘organ' is meant the nerve centre in the brain. The eyes and ears are only the instruments of seeing and hearing, and the organs are inside. If the organs are destroyed by any means, even if the eyes or the ears be there, we shall not see or hear.
So in order to control the mind, we must first be able to control these organs. To restrain the mind from wandering outward or inward, and keep the organs in their respective centres, is what is meant by the words Shama and Dama. Shama consists in not allowing the mind to externalize, and Dama, in checking the external instruments.
Source – From The Complete Works of Swami Vivekananda, (Kolkata: Advaita Ashrama, 2016), 1.395-6.