Self-assertion comes from thinking of the Seer and the instrument of vision as forming one self.

This is the fundamental idea of the Sankhya philosophy, of which the Yoga is avowedly the practical side. To translate this into our terms, we may say that the Seer is the spiritual man; the instrument of vision is the psychical man, through which the spiritual man gains experience of the outer world. But we turn the servant into the master. We attribute to the psychical man, the personal self, a reality which really belongs to the spiritual man alone; and so, thinking of the quality of the spiritual man as belonging to the psychical, we merge the spiritual man in the psychical; or, as the text says, we think of the two as forming one self.