People give highly mystic explanations of Karana Sharira as something exotic which is beyond the level of understanding and heavily fantasized. However, this is a completely bad interpretation. The definition is already given by Adi Shankaracharya in the book Tattvabodha.
Anirvācanyam-anādyavidyā rūpam sarvakāraṇam | Svātmasvarūpājñānam yat tatkāraṇaśarīram ||
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