प्रत्ययस्य परचित्तज्ञानम्।।

Patanjal Yoga-Sutra : vibhUtipad 19, 20

पातञ्जल योग-सूत्र : विभूतिपाद १९, २०

प्रत्ययस्य परचित्तज्ञानम्।।१९।।

(प्रत्ययस्य परचित्त-ज्ञानम्।।)

(न च तत् स आलम्बनं तस्य अविषयीभूतत्वात्।।२०।।

योग-सूत्र १९ / Aphorisms 19 points out that the प्रत्यय / expressions and gestures reveal about the state of one's चित्त / mind. प्रत्यय / pratyaya is therefore the mirror where-in a योगी yogi can see what is on the mind of someone else as if in a mirror, by one who can apply the  संयम / samyama on the प्रत्यय / pratyaya. In a way it is, kind of the reading another's mind.

(The present-day Body-language may be just a crude underdeveloped form of this only.) 

In the योग-सूत्र २० / Aphorism 20 it is further explained that though what is on someone's mind could be read in this way, the cause of the same couldn't be known, as that cause is not the विषय / object of the धारणा-ध्यान-समाधि-संयम or the holding and fixing the attention, contemplation and the rest -- samAdhi and samyama.

(Refer to the योग-सूत्र ४ / Aphorism 4 विभूतिपाद / vibhUtipada : त्रयमेकत्र संयमः।।४।।)

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