Yogācārabhūmi-śāstra, Volume 34 (84)

Original text:问。何因缘故。此三摩地名金刚喻。答。譬如金刚。望余一切末尼真珠。琉璃螺贝璧玉。珊瑚等诸珍宝。最为坚固。能穿能坏所余宝物。非余宝物所能穿坏。如是此三摩地。于诸有学三摩地中。最上最胜最为坚固。能坏一切所有烦恼。非上烦恼所能蔽伏。是故此三摩地名金刚喻。

Explanation: Question: For what reason is this samādhi named the "vajra simile"? Answer: Just as a vajra (diamond), compared with all other jewels such as maṇi gems, pearls, lapis lazuli, conch shells, jade, coral, and other precious treasures, is the most solid and durable—it can pierce and destroy all other jewels, while no other jewel can pierce or destroy it—so too this samādhi, among the samādhis of those still in training (śaikṣa), is the highest, most excellent, and most solid and durable. It can destroy all afflictions and cannot be obscured or subdued by the higher afflictions. Therefore this samādhi is named the "vajra simile."

From the first fruit to the third fruit, practitioners are all still in training (śaikṣa), because they still have afflictive outflows that have not been eliminated and still have dharma to cultivate. Since their afflictions are not yet exhausted, or they have not yet eliminated afflictions, they still have to cultivate and study in order to exhaust them. When afflictions are completely exhausted, one is an arhat of the fourth fruit, because there are no longer any outflows or afflictions to eliminate, no more dharma to cultivate, and one no longer needs to study the truths of the path of liberation—one has already reached ultimate purity and has already been liberated from all forms of existence in the three realms. Therefore this is called "beyond training" (aśaikṣa), whereas before the third fruit this is not the case. The samādhi attained at the third fruit is more excellent than those of the first and second fruits; it is the most solid and durable, and is called the vajra samādhi.