18:31 – The mind of the liberated one does not exert itself to be either meditative or active—it becomes meditative and active without any motive.
As the self, liberated people don’t exert themselves to be either meditative or active because they’re not the ego—the doer—the one that does the exerting (or at least claims to). As the actionless self, they’re ever free of the mind, whether it’s active or meditative.
18:32 – A dull-witted person becomes bewildered on hearing the real truth, but a sharp-witted person withdraws within himself like a dull person.
Here, “dull-witted person” refers to someone who’s not mentally prepared to study Vedanta and “sharp-witted person” refers to someone who is mentally prepared to study Vedanta. Ironically, both types of people have the same reaction to hearing the teaching: they become silent. Whereas the dull person becomes silent out of bewilderment, the sharp person is silent because they’ve drawn their attention inward to contemplate the meaning of the teaching.
18:33 – The ignorant constantly practice concentration and control of the mind. The wise, abiding in the real self, like persons in deep sleep, do not find anything to be done.
The ignorantly constantly practice concentration and control of the mind thinking that control of the mind is enlightenment. This is an idea that often appears in Vedanta circles owing to Vedanta’s close association with the practice of yoga. But the wise don’t find anything to be done because they know that no amount of mind control can turn them into the self. They can only recognize that they already are the self.
18:34 – The ignorant person does not attain peace either by inaction or action. The wise one becomes happy merely by knowing the truth.
The results of both action and inaction, being impermanent, can never lead to a lasting peace of mind. But self-knowledge can. Why? Because it shows you that you’re always the self, regardless of what the body-mind does or doesn’t do. As the self, you’re always at peace because your very nature is actionless, changeless and eternal.
18:35 – In this world those who devote themselves to diverse practices do not know the self, pure consciousness, which is complete, beloved, free from the ills of the body and untouched by the universe.
To devote yourself to diverse practices is to seek fulfillment in the world by doing certain actions and avoiding others. Because your attention is directed outwards to the body-mind and world, you can’t properly ‘turn inward’ to investigate your true nature and realize that it’s pure consciousness etc.