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Aura sensing, also known as Battle spirit detection, is an advanced form of extrasensory perception that grants the user the ability to perceive and interpret the aura or battle spirit emitted by their opponents.

Overview
Aura sensing can provide insights into the individual's emotions, intentions, or actions. By reading an opponent's aura or battle spirit, the user can gain a heightened awareness of their fighting style, strategies, and upcoming movements. It allows the user to determine the opponent's strength at a glance, predict where and how his opponents will attack, sense their location in his immediate surroundings, and accurately aim for their vitals and weak points.

To avoid the detection of one's aura, the sole method is to enter the Selfless State, wherein the user eradicates their battle spirit entirely, along with any traces of hatred, intent to fight, or desire to cause harm.

Muzan Kibutsuji
Using his aura sensing ability, Muzan could effortlessly discern the presence and strength of anyone, even from a considerable distance. This was evident when he detected the humans dispersed across the mountain, encircling the Ubuyashiki Estate. He could instantly sense that these humans were likely of Hashira strength.

There were two instances where Muzan's opponents successfully eluded his detection by concealing their battle spirits. When he encountered Yoriichi Tsugikuni, he felt that the swordsman looked weak and 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. Later, when he attempted to kill Kagaya Ubuyashiki, Muzan found himself incapable of detecting any trace of murderous intent within Kagaya. Instead, he perceived an aura reminiscent of a benevolent Buddha emanating from the man. It was only after the latter resorted to a self-immolation, sacrificing himself and his family to engulf Muzan in a fiery blaze, that he came to comprehend the profound depths of Kagaya's hatred and anger towards him. Muzan likened these emotions to a viper coiling within his dark, lifeless core. He stood in awe of Kagaya's exceptional skill in concealing his true intent, effectively evading his detection.

Tanjuro Kamado
Ten days before Tanjuro succumbed to his disease, there was an incident of a bear attacking and eating six people. The villagers hung ropes lined with Suzu bells on the mountain. When the bear appeared again and Tanjuro went to confront the bear, the animal was still on the other side of the rope. Even though the bells never rung, Tanjuro was able to sense the bear's presence and murderous intent while he was in his bed sleeping. It can be assumed that Tanjuro possessed the ability to detect aura's even outside of his immediate vicinity.

Yoriichi Tsugikuni
Despite never having met or seen Muzan Kibutsuji before, Yoriichi instantly recognized the man in front of him as the Demon Progenitor the moment they crossed paths for the first time. Using his Aura Sensing ability, he could sense that Muzan was different from other Demons. He was overflowing with violent vitality, which Yoriichi describes as closely resembling the magma blowing out of a volcano. It was bubbling and boiling, trying to swallow everything up.

Inosuke Hashibira
After the battle against Upper Moon Six, Inosuke explained and demonstrated to Tanjiro how he was able to notice Gyutaro's attacks before it actually happened. He perceives a distinct stinging sensation in the area where his opponent is directing their aim, regardless of whether it's from behind or not. Merely looking at him is enough for him to sense their presence instantly. Particularly when a murderous intent is present, his entire body experiences a tingling and prickling sensation. By honing the sensitivity of his skin, he can detect the aura of others that eludes ordinary vision. However, when the person nearby doesn't have any intent to kill, like when Hisa was just silently behind him, it will become hard for him to sense their aura. Inosuke thus suggests that the most effective way to elude detection would be to get close to your opponent without displaying any intention, ensuring that you remain unnoticed.