What is mind? From the yogic standpoint it is simply the individualized consciousness, the whole of it, the whole of your consciousness including your activities which the Western psychologist puts outside mind.
Only on the basis of Eastern psychology is Yoga possible. How shall we describe this individualized consciousness? First, it is aware of things. Becoming aware of them, it desires them. Desiring them, it tries to attain them.
So we have the three aspects of consciousness– intelligence, desire, activity. On the physical plane, activity predominates, although desire and thought are present. On the astral plane, desire predominates, and thought and activity are subject to desire. On the mental plane; intelligence is the dominant note, desire and activity are subject to it. Go to the buddhic plane, and cognition, as pure reason, predominates, and so on. Each quality is present all the time, but one predominates.
So with the matter that belongs to them. In your combinations of matter you get rhythmic, active, or stable ones; and according to the combinations of matter in your bodies will be the conditions of the activity of the whole of these in consciousness. To practice Yoga you must build your bodies of the rhythmic combinations, with activity and inertia less apparent. The yogi wants to make his body match his mind.