What exactly do we mean by the term ‘deep meditation’?
DhyAna Lakshana (sign of meditation) is mentioned very briefly in the following verse by Lord Shiva:
yathA nimilane kAle prapancha naiva pasyati |
Tathaiva unmilanehapi syAdetad dhyAnasya lakshanam ||
Just like the person, who has his eyes closed, can not see the outer world, in a similar manner, a person who is engrossed in meditation, can't see the same with even his eyes open - and this is the sign of DhyAnam.
KulArnavara Tantram 9.19
So, if a meditating person has kept his eyes open, but still he can not see anything around, then it is to be understood that he is properly meditating. So, a properly meditating person is not aware of the existence of the world around him.
And, when meditation reaches it's highest stage, that is called the SamAdhi stage and the signs of SamAdhi are slightly different from what is mentioned above.
Lord Shiva mentions the following signs:
Na shrinoti na chAghrAti na sprishati na pasyati |
Na jAnAti sukham dukkham na cha samlipyate manah ||
Na chApi kinchit jAnAti na cha vrudhyAti kAshthavat |
Evam shive vilinAtmA samAdistha ihochyate ||
One who does not hear, does not [perceive] smell, does not touch, does not see, does not feel either pain or pleasure, whose mind is not attached [to anything], who is just like a piece of wood, who does not understand or know anything --- completely merged in Shiva know such a person to be in SamAdhi.
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