Phra That refers to the physical remains of individuals who have attained enlightenment — meaning those whose minds have become pure and free from defilements through practicing the teachings of the Buddha. This can apply to both monks and laypeople. When the mind becomes completely pure and free from defilements, it develops a power that differs from a mind still clouded by defilements. This power of purity gradually transforms various parts of the body, giving them inner qualities distinct from those of an ordinary person. These qualities cannot be seen from the outside; they become visible only after the person’s body is cremated following death.
If a person who has practiced the Dhamma according to the Buddha’s teachings attains enlightenment at a preliminary stage — such as Sotāpanna (Stream-Enterer), Sakadāgāmī (Once-Returner), or Anāgāmī (Non-Returner) — and passes away before attaining the highest Dhamma, which is Arahantship, and subsequently is reborn in a heavenly realm where they then attain the higher Dhamma, in such a case no Phra That will appear.
Phra That as commonly known can be divided into two types: the relics of the Buddha, commonly referred to as Phra Borom Saririk That (the Buddha’s sacred bodily relics), and the relics of the Arahants, commonly referred to as Phra Arahanta That.
The relics of the Buddha are the parts of his physical body that became relics, divided into the solid parts — namely the bones — and the soft parts — namely the flesh, skin, and all internal organs. That is to say, the entirety of the Buddha’s body transforms into relics.
The relics of the Arahants are the bodily remains of the Buddha’s disciples who attained the highest Dhamma in the same manner as the Buddha. These may be monks, novices, laymen, or laywomen. The relics of Arahants vary: in some cases, only the bones become relics, while in others, both the bones and certain soft tissues or internal organs become relics. This depends on each individual’s accumulated merit (pāramī) and their proficiency in attaining the meditative absorptions (jhāna-samāpatti).
Credit: Thongdee Hansakhunaram, (2002). Phra Borom Saririk That and Phra Arahanta That, Phra Borommathat Foundation under Royal Patronage of the Supreme Patriarch, Siriwattana Interprint Co., Ltd. (Public Company).