The Yogācārabhūmi-śāstra: Volume Thirty-Four (84 fascicles)

Question: For what reason is this samadhi named the Vajra Simile?

Answer: It is like how the vajra, compared to all other jewels such as mani pearls, genuine pearls, vaidūrya, conch shells, jade, coral, and so forth, is the most steadfast. It can penetrate and destroy all other precious substances, while no other jewel can penetrate or destroy it. Similarly, this samadhi, among all the samadhis of those still in training, is the highest, most supreme, and most steadfast. It can destroy all defilements, and cannot be obscured or subdued by superior defilements. Therefore, this samadhi is named the Vajra Simile.

Explanation:

Question: What is the reason this samadhi is metaphorically named after the vajra?

Answer: It is like the vajra, which, when compared to all other jewels such as mani pearls, genuine pearls, vaidūrya, conch shells, jade, coral, and so on, is the most steadfast—able to penetrate and destroy all other precious objects yet cannot itself be penetrated or destroyed by them. Such is this samadhi: among all the samadhis of those still in training, it is the highest, most supreme, and most steadfast. It can destroy all defilements and cannot be obscured or subdued by superior defilements. Hence, this samadhi is metaphorically named after the vajra.

From the first fruition to the third fruition, practitioners are all in the stage of "still in training" because they have not yet eradicated all defilements and outflows, and there are still dharmas to be cultivated. Since their defilements have not been fully severed—or not severed at all—they still engage in cultivation to exhaust all defilements. The complete eradication of defilements is attained at the fourth fruition of Arhatship, for at that stage, there are no more outflows or defilements to sever, no dharmas left to cultivate, and no need to further study the truths of the path to liberation. Having reached ultimate purity and liberation from all existences within the three realms, they are called "beyond training." However, this is not the case for those prior to the third fruition. The samadhi attained at the third fruition is more supreme than those of the first and second fruitions—it is the most steadfast and is named the Vajra Samadhi.