Turn from him to the seeker for the Self through the Not-Self. This is the way of the scientist, of the man who uses the concrete, active Manas, in order scientifically to understand the universe; he has to find the real among the unreal, the eternal among the changing, the Self amid the diversity of forms. How is he to do it? By a close and rigorous study of every changing form in which the Self has veiled himself. By studying the Not-Self around him and in him, by understanding his own nature, by analysing in order to understand, by studying nature in others as well as in himself, by learning to know himself and to gain knowledge of others; slowly, gradually, step by step, plane after plane, he has to climb upwards, rejecting one form of matter after another, finding not in these the Self he seeks. As he learns to conquer the physical plane, he uses the keenest senses in order to understand, and finally to reject. He says: “This is not my Self. This changing panorama, these obscurities, these continual transformations, these are obviously the antithesis of the eternity, the lucidity, the stability of the Self. These cannot be my Self.” And thus he constantly rejects them. He climbs on to the astral plane and, using there the finer astral senses, he studies the astral world, only to find that that also is changing and manifests not the changelessness of the Self. After the astral world is conquered and rejected, he climbs on into the mental plane, and there still studies the ever-changing forms of that Manasic world, only once more to reject them: “These are not the Self.” Climbing still higher, ever following the track of forms, he goes from the mental to the Buddhic plane, where the Self begins to show his radiance and beauty in manifested union. Thus by studying diversity he reaches the conception of unity, and is led into the understanding of the One. To him the realisation of the Self comes through the study of the Not-Self, by the separation of the Not-Self from the Self. Thus he does by knowledge and experience what the other does by pure thinking and by faith. In this path of finding the Self through the Not-Self, the so-called Siddhis are necessary. Just as you cannot study the physical world without the physical senses, so you cannot study the astral world without the astral senses, nor the mental world without the mental senses. Therefore, calmly choose your ends, and then think out your means, and you will not 'be in any difficulty about the method you should employ, the path you should tread.
Thus we see that there are two methods, and these must be kept separate in your thought. Along the line of pure thinking — the metaphysical line — you may reach the Self. So also along the line of scientific observation and experiment — the physical line, in the widest sense of the term physical — you may reach the Self. Both are ways of Yoga. Both are included in the directions that you may read in the Yoga Sutras of Patanjali. Those directions will cease to be self-contradictory, if you will only separate in your thought the two methods. Patanjali has given, in the later part of his Sutras, some hints as to the way in which the Siddhis may be developed. Thus you may find your way to the Supreme.