In plain words
The conversations in the mind are the root cause of all sorrow
Two kinds of thinking go on in the Manas. One is reflection; the second is conversation in the mind. Reflections are what bring us to the state of solving our problems. Conversations in the mind are what make problems grow further and lead man down many kinds of wrong path. So the two behave quite differently. Into these conversations in the mind we bring the faults of others, evil, and circumstance. These conversations in the mind are the root cause of the ill health we have, of the old age that comes on us early, and of all sorrow. They spread darkness in the Manas like black clouds. These conversations grow and swell until they make us forget even our humanity. Because a conversation lived through in the waking state starts up again in the dream state, we lose the rest of the mind in sleep and become weaklings unable to check pain, sorrow and grief. No result at all is gained from them. The man who can halt these conversations in the mind and attain rest is the complete man.