Selfless State

The Selfless State (の), or the Realm of the Highest/Domain of Supremacy, is a mysterious state of being implied to be the zenith of physical combat.

Overview
The Selfless State is an esoteric and mysterious state of being in which one completely erases their Fighting Spirit, will to fight, bloodlust, anger, hatred, malevolence, drive and animosity. Once in this state, the user's behavior becomes incredibly tranquil and focused, often being compared to that of a plant.

The existence of this state of being remains shrouded in obscurity, known only to a limited few with profound understanding, and even fewer possess the capability to attain it. Throughout his centuries-long existence, Akaza has never encountered a human devoid of a Fighting Spirit, an impossibility since even infants emit a faint trace of it.

In Combat
Although it does not enhance one's own combat prowess, it is implied to be regarded as the the highest state of physical combat one can achieve. It is extremely effective against high-level combatants who rely on various forms of extrasensory perception to detect malice, aura or Fighting Spirit. The incapacity to perceive the battle spirit of an individual in the Selfless State induces sensory confusion, resulting in a disorienting experience akin to battling against an inconspicuous intruder, like an out-of-place plant.

By combining the Selfless State with the Transparent World, one could strike a decisive and lethal blow by anticipating the vital points and movements of their opponent through slow-motion perception and precognition, as well as skillfully evading detection of their own attacks.

Unlocking
Based on information Tanjuro Kamado shared with Tanjiro, one must engage their five senses and cultivate an acute awareness of their entire being, down to the tiniest blood vessel. By focusing on precise breath control, movements, and sensations, they can unleash maximum strength with minimal effort. As a result, the mind transcends visibility and enables the closure of all senses, leading to a state of complete detachment from oneself.

Initially, maintaining a Selfless State even for a few seconds proves to be a formidable challenge, as one will struggle and suffer from the strength being at its limits, as evident in Tanjiro's case when he entered the Selfless State for the first time. However, through diligent training, unwavering dedication, and continuous refinement, one can gradually extend the duration. With unwavering commitment, Tanjuro was able to sustain this state throughout his entire lifetime.

Yoriichi Tsugikuni
When Yoriichi encountered Muzan Kibutsuji for the first time, he instantly realized the purpose of his life is to kill the Demon Progenitor, and was filled with unwavering determination to accomplish this. Yet, in Muzan's retelling of the encounter, he felt that the swordsman looked weak. He could not detect any ambition, willpower, hatred, and killing intent in him. Because of that, Muzan underestimated him and was totally stunned when Yoriichi easily decapitated and slashed him, giving him wounds that continued to burn his flesh for over hundreds of years.

Tanjuro Kamado
In Tanjiro's memories of his father, Tanjuro remained perpetually in the Selfless State. He was like a plant, always gentle and would never have any emotional outbursts, which never changed in the slightest until his death. His voice was as calm as a murmuring river and he always had a soft smile. Ten days before he succumbed to his death, he demonstrated to Tanjiro how to kill a bear while remaining in the Selfless State. Tanjiro sensed how his father's smell never wavered, there was no hesitation or fear in him before or after cutting off the bear's head, nor was there any intent to kill. Tanjuro still maintained his usual plant-like presence the entire time.

Tanjiro Kamado
In the final battle against Akaza, Tanjiro noticed how accurately Akaza could anticipate and react to attacks, even when they originated from behind or from blind spots. Recalling Akaza's remarks about Kyojuro Rengoku's battle spirit and Inosuke Hashibira's explanation, Tanjiro deduced that Akaza must possess the ability to sense his opponent's fighting spirit through his compass technique. Drawing upon the insights Tanjuro provided him with during his childhood, Tanjiro entered the Selfless State, rendering his presence entirely undetectable by Akaza. Through the combined utilization of the Selfless State and Transparent World, he then successfully severed Akaza's head. Bewildered by his inability to perceive Tanjiro's fighting spirit, Akaza likened the experience to engaging in combat against a mere plant. He saw a distinct absence of hatred or anger in the boy's eyes, instead finding within them an aura of serene tranquility — a realm of existence that Akaza had tirelessly pursued throughout centuries of martial arts practice.

Due to Tanjiro's lack of experience in regulating his strength while in the Selfless State, he found himself suddenly flooded by an intense surge of agonizing pain and overwhelming fatigue immediately after beheading Akaza. His eyes throbbed intensely, his muscles grew numb, and he swiftly lost his grasp on the Selfless State. When the headless body of Akaza unleashed his compass technique, he instantly sensed Tanjiro's presence once more and delivered a forceful kick that sent Tanjiro flying into a wall, causing him to lose consciousness.

Trivia

 * The Selfless State is also often referred to as the  State of Anattā, a state of nothingness where one is so focused on the fight that they become completely detached from everything else and can engage in combat without wasteful thoughts and movements. The nothingness one experiences in this state is akin to plants on a battlefield, where opponents are always on the constant lookout for moving targets while presenceless plants in the background are usually completely ignored. Thus, to be in the State of Anatta is to "be" the plants on the battlefield, to mesh oneself in the background so much until their opponents don't notice them until it's too late.
 * Kagaya Ubuyashiki concealing his intense animosity and hatred towards Muzan Kibutsuji while maintaining a calm and beatific demeanour was worth noting, but due to inconclusive evidence, it is unclear whether he entered the Selfless State.

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