Kshana is accepted as a reality in which everything has its existence, persistence and finally dissolution. As per Yoga Sutra, “Therefore, the present indeed, the singleness of the moment, is Kshana.” (Yogasutra III. 52.)
In order to clarify the concept Vyasa, the commentator of Yoga Sutra further states that a small portion of material can be divided into half and further sub-divided continuously until there remains only the atom. That indivisible, extremely minute unit of a substance is known as Paramanu. So Kshana is the minutest available indivisible unit of time (Pramapakrsha paryantah kalah kshanah) (Yogasutra III. 52.)
To the Vaisesika thinkers, the atom or paramanu is the basis of the superstructure of matter; but yet according to the doctrine, parimandalya (term used by Raghunatha Siromani – a logician of the 16th century, i.e. paramanu) is not accepted as the cause of matter; the triad trasarenu is the cause.
In the same way, Kshana should not be thought of as subtle unit of time, for time is not something real, but a mode of thought, buddhinirmana.
Kshana is defined by Vyasa (Yogasutra III. 52.) as the time that is a requirement for a paramanu to leave its former spot and occupy the later one. It flows on in an endless sequence.
Raghunatha Siromani in Atmatattvaviveka accepts Kshana as a time unit. The time that is required for the blinking of the eyes is called nimesha. Kshana is the fourth part, i.e. one-fourth of nimesha.
The commentator emphatically states that aggregation of Kshana is not possible, for two Kshanas cannot meet together.
The commentator, in order to emphasize this truth, says that kshana, and its sequence do not accept any addition (samahara) of substances; rather, some of the commentators agree that addition is conceptual and not actual as is also the supposed division of time known by terms like muhurta and ahoratra (the span of the day and night).
In the doctrine designated as Trika Kshana, Kshana is a very subtle time unit. So long as a particular concept, which is very subtle by nature, persists, there is kshana (trika kshana VII.25).
A yogi is able to arrive at the point where there is nothing else than consciousness itself, which is free from thought constructs. Kshana definitely belongs to time, but is partless and cannot be broken (abhedya nirmaso kalalakshana).
The yogi makes an effort to arrive at the eternal moment (kshana) by making the mind so subtle that it may be possible for him to consume time itself.
The state where time no longer exists is known as kala sankarshana sthiti, the finest or sublets state of time. It is known as Eternal Now. It is also the state where the seeker of the truth become one with the Divine, where the question of space and time ends.
Bibliography –
The Yogasutra of Patanjali (1924) – B. D. Basu – Published by Panini Office Allahabad.
Encyclopedia Of Hinduism – Volume VI – page 142 – 143 - IHRF