The Yogi proposes to attain that fine state of perception in which he can perceive all the different mental states. There must be mental perception of all of them. One can perceive how the sensation is travelling, how the mind is receiving it, how it is going to the determinative faculty, and how this gives it to the Purusha. As each science requires certain preparations and has its own method, which must be followed before it could be understood, even so in Raja-Yoga.