The cause of conjunction is ignorance.

Bryant Commentary:

Vācaspati Miśra and Vijñānabhikṣu elaborate somewhat on the fourth possible cause of ignorance outlined in the previous sūtra. Creation in Hindu cosmology is cyclical. At the end of each cosmic cycle, all manifest reality, the world and the evolutes of prakṛti, dissolve back into their original source matrix along with the souls in saṁsāra—the puruṣas who have not attained liberation—and remain there latent and inactive until the next cosmic cycle begins anew. This primordial soup, called pradhāna, thus contains all the saṁskāras from all the cittas of all the individual puruṣas that had not had a chance to fructify during the last cycle.46 At the beginning of the new cycle, these saṁskāras reactivate and cause pradhāna to produce an individual citta for each puruṣa appropriate to the specific saṁskāras possessed by that same puruṣa at the end of the last cycle. The puruṣa is thus like a fish trapped in a net of its previous saṁskāras and karma, says Rāmānanda Sarasvatī. As a result of the puruṣa being reconnected with a citta, its previous saṁskāras, most notably the saṁskāra of ignorance (i.e., the misidentification between the puruṣa and prakṛti), reexert their influence. In other words, the puruṣa picks up where it left off. The point is, from this perspective, that it is the saṁskāras that cause ignorance. This cycle of creation and dissolution is eternal for the Yoga school until liberation occurs (saṁsāra has no beginning, but it has an ending). Since the eternality of this cycle is axiomatic, the Yoga school avoids having to account for any primordial saṁskāra of ignorance that may have activated the whole cycle in the first place.

When intelligence contains the saṁskāras of ignorance, says Vyāsa, it remains active in the realm of prakṛti and thus does not produce discrimination about the true nature of puruṣa. Saṁskāras impel the intelligence to perform the first of its two functions, as expressed in II.18, namely, to provide experience of prakṛti, and it is this that is the cause of bondage. Intelligence ceases its activity only when it has attained its alternative and ultimate function, which is to provide discrimination about the distinction between puruṣa and prakṛti. As was discussed in some detail in I.50, the saṁskāra of discrimination overpowers all other saṁskāras. When this happens, ignorance, avidyā, the cause, hetu, of bondage, is removed, and ignorance, we recall, is the support of the other kleśas, obstacles (II.3–4), so they, too, dissolve.
In other words, complete liberation occurs only when intelligence first provides discrimination and then ceases to act altogether. Although discrimination, a function of buddhi, is initially indispensable in attaining the goal of yoga, as long as it remains active, puruṣa is still connected with buddhi, and thus complete liberation is not realized. But discrimination eventually completely destroys ignorance and thus its own base, like fire destroys its own fuel, says Hariharānanda. This results in asamprajñāta-samādhi, the final goal of yoga.

One might argue, says Vyāsa, that this claim that full liberation occurs only after discrimination has dissolved itself is rather like an impotent man who, when asked by his wife why she does not have children as her sister has children, replies that he will beget children in her after he is dead. If intelligence cannot provide liberation while it is alive and active, why should one believe that it will do so after it becomes lifeless and inactive? Vyāsa affirms, again, that full and final liberation occurs precisely when the intelligence ceases to act. Intelligence ceases to act when ignorance is removed. And ignorance is removed by knowledge. In other words, bondage is caused by ignorance, ignorance is removed by knowledge, the discriminatory aspect of intelligence, and then intelligence, having performed its grand finale, ceases to operate, and the full freedom of puruṣa occurs. Thus, intelligence and knowledge are not the direct cause of liberation, but by removing ignorance, they are the indirect cause.

Avidya Or Nescience As Its Cause.

The cause of conjunction is ignorance.

Bryant Commentary:

Vācaspati Miśra and Vijñānabhikṣu elaborate somewhat on the fourth possible cause of ignorance outlined in the previous sūtra. Creation in Hindu cosmology is cyclical. At the end of each cosmic cycle, all manifest reality, the world and the evolutes of prakṛti, dissolve back into their original source matrix along with the souls in saṁsāra—the puruṣas who have not attained liberation—and remain there latent and inactive until the next cosmic cycle begins anew. This primordial soup, called pradhāna, thus contains all the saṁskāras from all the cittas of all the individual puruṣas that had not had a chance to fructify during the last cycle.46 At the beginning of the new cycle, these saṁskāras reactivate and cause pradhāna to produce an individual citta for each puruṣa appropriate to the specific saṁskāras possessed by that same puruṣa at the end of the last cycle. The puruṣa is thus like a fish trapped in a net of its previous saṁskāras and karma, says Rāmānanda Sarasvatī. As a result of the puruṣa being reconnected with a citta, its previous saṁskāras, most notably the saṁskāra of ignorance (i.e., the misidentification between the puruṣa and prakṛti), reexert their influence. In other words, the puruṣa picks up where it left off. The point is, from this perspective, that it is the saṁskāras that cause ignorance. This cycle of creation and dissolution is eternal for the Yoga school until liberation occurs (saṁsāra has no beginning, but it has an ending). Since the eternality of this cycle is axiomatic, the Yoga school avoids having to account for any primordial saṁskāra of ignorance that may have activated the whole cycle in the first place.

When intelligence contains the saṁskāras of ignorance, says Vyāsa, it remains active in the realm of prakṛti and thus does not produce discrimination about the true nature of puruṣa. Saṁskāras impel the intelligence to perform the first of its two functions, as expressed in II.18, namely, to provide experience of prakṛti, and it is this that is the cause of bondage. Intelligence ceases its activity only when it has attained its alternative and ultimate function, which is to provide discrimination about the distinction between puruṣa and prakṛti. As was discussed in some detail in I.50, the saṁskāra of discrimination overpowers all other saṁskāras. When this happens, ignorance, avidyā, the cause, hetu, of bondage, is removed, and ignorance, we recall, is the support of the other kleśas, obstacles (II.3–4), so they, too, dissolve.
In other words, complete liberation occurs only when intelligence first provides discrimination and then ceases to act altogether. Although discrimination, a function of buddhi, is initially indispensable in attaining the goal of yoga, as long as it remains active, puruṣa is still connected with buddhi, and thus complete liberation is not realized. But discrimination eventually completely destroys ignorance and thus its own base, like fire destroys its own fuel, says Hariharānanda. This results in asamprajñāta-samādhi, the final goal of yoga.

One might argue, says Vyāsa, that this claim that full liberation occurs only after discrimination has dissolved itself is rather like an impotent man who, when asked by his wife why she does not have children as her sister has children, replies that he will beget children in her after he is dead. If intelligence cannot provide liberation while it is alive and active, why should one believe that it will do so after it becomes lifeless and inactive? Vyāsa affirms, again, that full and final liberation occurs precisely when the intelligence ceases to act. Intelligence ceases to act when ignorance is removed. And ignorance is removed by knowledge. In other words, bondage is caused by ignorance, ignorance is removed by knowledge, the discriminatory aspect of intelligence, and then intelligence, having performed its grand finale, ceases to operate, and the full freedom of puruṣa occurs. Thus, intelligence and knowledge are not the direct cause of liberation, but by removing ignorance, they are the indirect cause.

Not seeing things as they are is the cause of this phenomenon.

Ignorance of the True Self is the cause of this illusory union.

Its cause is the lack of awareness of his Real nature.

Moral injunctions (yama), fixed observances (niyama), posture (Asana), regulation of breath (pranayama), internalization of the senses towards their source (pratyahara), concentration (dharana), meditation (dhyana) and absorption of consciousness in the self (samadhi), are the eight constituents of yoga.

Iyengar Commentary:

This sutra sets out the eightfold path of yoga (astanga yoga), which Patañjali proceeds to describe in detail in the remaining sutras of sadhana pada and in the first three sutras of vibhuti pada.

Restraints and observances that are bound by tradition and lineage follow uninterruptedly in the practice of yoga. Although Asana, pranayama and pratyahara are separate entities, they depend upon one another for expressing the hidden facets of yoga. These stages, which enable the seeker to rise in the art of yoga, are called progressive sadhana. Through them we reach higher and higher. The first five aspects of yoga are individual efforts for the evolution of the consciousness, while dharana, dhyana and samadhi are the universal manifestation or the natural states of yoga (yoga svarupa).

The cause of this union is ignorance.

Satchidananda Commentary:

Here, Patañjali laughs at the idea he has just expressed. The cause of the saṁyoga is ignorance. This may seem a bit confusing, but if we understand it properly there’s no puzzle. You see, in the previous sūtra, we’re still in the world and wondering about the reason for nature. Once the Puruṣa understands itself, it thinks, “How did this union come about? It’s because I’ve forgotten myself. What an ignorant person I was. Because of my ignorance I created this union.” Such a person laughs at it, but this attitude comes only after realization. It’s like a dreaming person who, upon waking, laughs at his or her own frightening dream. The understanding behind this sūtra is a result of realization. Once we realize, we can advise others: “I was ignorant. I had terrible experiences. I thought nature was real, happiness was real. I ran after them. But now I know what they are. I learned the hard way. Do you also want to have to learn the hard way? Why don’t you take my advice?”

These sūtras are reminiscent of the Four Noble Truths of the Buddha: the misery of the world, the cause of misery, the removal of that misery and the method used to remove it. Patañjali tells us that pain can be avoided. He further tells us that its cause is ignorance. In sūtra 26, he gives us another word, hāna, the removal of this misery, and then hānopāya, the method to remove it. We can really see the similarity between the Four Noble Truths and the Yoga Sutras. We needn’t search for who copies whom. Truth is the same always. Whoever ponders it will get the same answer. The Buddha got it. Śri Patañjali got it. Lord Jesus got it. Prophet Muhammad got it. The answer is the same, but the method of working it out may vary this way or that.

Ignorance is its cause.

SV Commentary:

Through ignorance we have joined ourselves with a particular body, and thus opened ourselves to misery. This idea of body is a simple superstition. It is superstition that makes us happy or unhappy. It is superstition caused by ignorance that makes us feel heat and cold, pain and pleasure. It is our business to rise above this superstition, and the Yogi shows us how we can do this. It has been demonstrated that, under certain mental conditions, a man may be burned, yet, while that condition lasts, he will feel no pain. The difficulty is that this sudden upheaval of the mind comes like a whirlwind one minute, and goes away the next. If, however, we attain it scientifically, through Yoga, we shall permanently attain to that separation of Self from the body. 

The effective cause, however, of the conjunction of the individual consciousness with its own Will-to-be is Nescience, which means the potency of the habit of unreal cognition. The Will-to-know, possessed as it is of the aroma of the habits of unreal cognition, does not culminate into the knowledge of the self, which is the end of its work, and thus

having still a duty to perform, comes back. When, however, it reaches the culmination of its work, which is the attainment of the knowledge of the Puru§a, its work is achieved, ignorance is gone, the cause of bondage no longer remains, and it does no longer come back.

Someone ridicules this position by the story of the impotent husband. A foolish wife thus addressed her husband : — “ My dear, my sister has got children. Wherefore have I none ? ” He said to her : — “ I shall beget children unto thee when I am dead.’* Similarly, this knowledge while in existence, does not cause the mind to cease from action ; what hope is there that it will cause cessation when suppressed ? Says on this subject a* teacher very nearly perfect: — “Is not Moksa (freedom) the cessation of the Will-to-be itself and nothing else ? The Will-todbe ceases to act when the cause of ignorance no longer remains. The Ignorance which is the cause of bondage is removed by knowledge. Moksa (perfect freedom) then is only the cessation of the mind from its work. Wherefore then this mental confusion out of place ?