The Mind's Eye (心眼 Shingan?) is a Samurai's ability to perceive their surroundings on a spiritual level. This abiility allows them to view the Samurai Souls of others and judge their purity.
As suggested by Hagamichi, all Samurais can learn this ability.
Samurai Eight: Hachimaruden Manga; Chapter 23, page 13
Samurai Eight: Hachimaruden Manga; Chapter 8, page 8