If men were satisfied with indulging in reason, memory and imagination, the purification of the Buddhi and the development of the higher faculties would be an easy matter. But there is another means of thought which they habitually indulge in and that is Manas.
The Manas is a receptive organ; it receives the images expressed on the eye, the ear etc., and turns them into things perceived. Besides, it receives the ideas, images etc., sent down from the Vijnana into the Chitta and passes them on to the latter organ.
In this passage these things become what are called concepts, that is, things conceived or thought of.
Percept and concept together make what is called understanding. Many believe that concepts are merely percepts put together and converted into what is called thought. According to this idea, all thought is merely the arrangement of sensation in the terms of language. Even when I talk of abstract qualities, for example, virtue, courage etc., I am not thinking of anything beyond sensation, but merely a classification of virtuous and courageous sensations and actions put together and labelled with the name virtue or courage.
When the Yogic power is developed. This single fact that man can see with his Buddhi the truth about a thing he has never seen or known before, is enough to destroy the materialistic idea of thought.
The Manas responds to the senses and is always forming percepts and concepts about the sensations it receives. These ideas it sometimes gets from the outside world, sometimes from the passive memory in the Chitta, sometimes from the Buddhi. But it tries to impose them all on the Buddhi. It tests everything which it does not take for granted by reference to the senses. "I saw that", "I heard that", therefore it is true, that is the reasoning of the Manas.
What are we to do with the Manas? Get it to be still, says the Yogin.