They would experience permanent cessation of suffering with no further rebirth, though describing it remains difficult.
A person who has attained nibbana (nirvana) would know with certainty because the attainment is irreversible and permanent. According to the Pali Canon, once nibbana is reached, there is no possibility of returning to ignorance or creating fresh karma that leads to rebirth. The Buddha taught that nibbana represents the complete cessation of greed, hatred, and delusion—the three roots of suffering—and this cessation cannot be undone.
The most direct way a person would know is through direct experience of this permanent shift in consciousness. The Dhammapada describes it as the "unshakeable liberation of mind." Unlike temporary meditative states that fade when meditation ends, nibbana is an irreversible transformation of one's relationship to existence itself.
The attainment of nibbana manifests as the complete end of tanha (craving or thirst). This isn't merely reduced desire—it's the permanent elimination of the mental drive that perpetuates suffering and rebirth. A person who has attained nibbana would directly know that the fundamental dissatisfaction underlying existence has vanished.
In the Samyutta Nikaya, the Buddha explains that nibbana is the cessation of the five aggregates (form, feeling, perception, mental formations, and consciousness) that normally constitute a sense of self. Someone experiencing this would recognize the absence of the compulsive mental habits that normally drive craving and aversion. There would be no experience of "I want" or "I don't want" arising from ignorance.
Importantly, the Buddha acknowledged the difficulty of describing nibbana to others. In the Udana, he states that nibbana is "hard to see, hard to understand," and its nature cannot be easily communicated through words. Someone who attained it would know directly, but explaining that knowledge to others presents a fundamental problem: nibbana transcends conventional concepts of existence, non-existence, time, and self.
This is why the Buddha often described nibbana negatively—as the absence of greed, hatred, and delusion; the cessation of the five aggregates; the end of the cycle of rebirth. The positive experience itself, he suggested, is beyond conceptual description. A practitioner would recognize it through direct experience rather than through rational analysis.
Theravada Buddhism, which emphasizes the Pali Canon, teaches that an arahant (one who has attained nibbana) experiences the permanent end of rebirth immediately upon attainment. They would know because the compulsive process of creating karma ceases entirely.
Mahayana Buddhism sometimes describes enlightenment (bodhi) differently, emphasizing the realization of Buddha-nature or emptiness, though ultimate nibbana remains central. Some Mahayana schools suggest that the distinction between nibbana and samsara (the cycle of rebirth) is ultimately illusory. However, the direct knowing of liberation itself—the irreversible cessation of suffering—remains the common thread.
The Pali Canon records numerous accounts of individuals attaining nibbana, from the Buddha's first disciples to lay followers. These accounts typically describe specific moments of insight followed by declarations of attainment. Sariputta, for example, is described as attaining the first stage of enlightenment through a single verse heard from another monk.
These accounts suggest that attainment involves a clear, unmistakable shift in consciousness—something the person recognizes with absolute certainty. The famous declaration "Destroyed is birth, lived is the holy life, done is what needed to be done, there is nothing further for this world" reflects this certainty. While we cannot verify another's attainment, the texts consistently portray it as a self-evident realization rather than a matter of belief.