Mystic Eyes of Death Perception

The eyes possessed by Tohno Shiki, that can see the death of things.
This death is expressed as black lines and points. However, the lines are nothing more than places where a substance breaks easily, while points are death itself.
If a line is cut, then even if the main body survives, that particular part will die, never to move again.
If a point is pierced, then the individual will stop functioning completely.
Though they are said to see the form of death, strictly speaking it's more accurate to say they're reading the lifespan of a substance—the existential limit determined at the moment of creation.
Now, this was already covered in the main story of Tsukihime, but lines cannot be perceived on things that can't be killed (destroyed) in the current world. It's probably because as a human, Shiki's point of reference conforms to the limits of the humanity of his era.
If he tries to perceive the death of things that exceed those limits, Shiki himself will begin to break down.
Originally, Shiki's eyes were meant to “see that which cannot be seen.” However, after having two near death experiences he was somehow connected to (the Root), and his brain began to be able to comprehend death itself. Or to put it in other words, it is only by virtue of his mystic eyes and brain working together as a set that he is able to perceive death.
By the way, Shiki's eyes turn blue when he intentionally tries to see death. Seeing as how blue is the color of pure eyes, it's likely that Shiki's eyes are also of that type.

Tsukihime Data Collection: Tsukihime Dictionary
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The eyes possessed by Tohno Shiki, that can see the death of things.
This death is expressed as black lines and points. The lines are places where a substance breaks easily, and the points are death itself. If a line is cut, then even if the main body survives, that particular part will die, never to move again. If a point is pierced, then the individual will stop functioning completely. Also, while lines can be seen without consciously trying, seeing points, the source of those lines, requires extreme concentration.
Though they are said to see the form of death, strictly speaking it's more accurate to say they're reading the lifespan of a substance—the existential limit determined at the moment of creation. Now, this was already covered in the main story of Tsukihime, but lines cannot be perceived on things that can't be killed (destroyed) in the current world. It's probably because as a human, Shiki's point of reference conforms to the limits of the humanity of his era.
Originally, Shiki's eyes were meant to “see that which cannot be seen.” However, after having two near death experiences he was somehow connected to (the Root), and his brain began to be able to comprehend death itself. Or to put it in other words, it is only by virtue of his mystic eyes and brain working together as a set that he is able to perceive death.
By the way, Shiki's eyes turn blue when he intentionally tries to see death. Seeing as how blue is the color of pure eyes, it's likely that Shiki's eyes were originally of that type.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
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The eyes possessed by Tohno Shiki, that can see the death of things.
A unique ability that allows the user to grasp the death of living beings as points and lines. Stabbing or cutting these will ignore things like vital activity and kill the target directly.
Though they are said to see the form of death, strictly speaking it's more accurate to say they're reading the lifespan of a substance—the existential limit determined at the moment of creation.
Lines cannot be perceived on things that can't be killed (destroyed) in the current world. It's probably because as a human, Shiki's point of reference conforms to the limits of the humanity of his era.
Originally, Shiki's eyes were meant to “see that which cannot be seen.” However, after having two near death experiences his brain began to be able to comprehend death itself. Or to put it in other words, it is only by virtue of his mystic eyes and brain working together as a set that he is able to perceive death.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary