Before proceeding further, I will tell you a little of the Sânkhya philosophy, upon which the whole of Raja-Yoga is based. According to the Sankhya philosophy, the genesis of perception is as follows: the affections of external objects are carried by the outer instruments to their respective brain centres or organs, the organs carry the affections to the mind, the mind to the determinative faculty, from this the Purusha (the soul) receives them, when perception results. Next he gives the order back, as it were, to the motor centres to do the needful. With the exception of the Purusha all of these are material, but the mind is much finer matter than the external instruments. That material of which the mind is composed goes also to form the subtle matter called the Tanmâtras. These become gross and make the external matter. That is the psychology of the Sankhya. So that between the intellect and the grosser matter outside there is only a difference in degree. The Purusha is the only thing which is immaterial. The mind is an instrument, as it were, in the hands of the soul, through which the soul catches external objects. The mind is constantly changing and vacillating, and can, when perfected, either attach itself to several organs, to one, or to none.