Sri Aurobindo
Letters on Yoga
4. Reason, Science and Yoga
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The interpenetration of the planes is indeed for me a capital and fundamental part of spiritual experience without which yoga as I practise it and its aim could not exist. For that aim is to manifest, reach or embody a higher consciousness upon earth and not to get away from earth into a higher world or some supreme Absolute. The old yogas (not quite all of them) tended the other way – but that was, I think, because they found the earth as it is a rather impossible place for any spiritual being and the resistance to change too obstinate to be borne; earth-nature 
looked to them in Vivekananda’s simile like the dog’s tail which, every time you straighten it, goes back to its original curl. But the fundamental proposition in this matter was proclaimed very definitely in the Upanishads which went so far as to say that Earth
is the foundation and all the worlds are on the earth and to imagine a clean-cut or irreconcilable difference between them is ignorance: here and not elsewhere, not by going to some other world, the divine realisation must come. This statement was used to justify a purely individual realisation, but it can equally be the basis of a wider endeavour.
Current publication:
Sri Aurobindo. Letters on Yoga // SABCL.- Volume 22. (≈ 28 vol. of CWSA).- Pondicherry: Sri Aurobindo Ashram, 1971.- 502 p.
Other publications:
[Largest or earliest found passage: ] Sri Aurobindo. On Himself // SABCL.- Volume 26. (≈ 35 vol. of CWSA)
Sri Aurobindo. Letters on Yoga. I // CWSA.- Volume 28. (≈ 22 vol. of SABCL).- Pondicherry: Sri Aurobindo Ashram, 2012.- 590 p.
Sri Aurobindo. Letters of Sri Aurobindo: In 4 Series.- First Series [On Yoga].- Bombay: Sri Aurobindo Sircle, 1947.- 416 p.